THE  NATURE  OF  MATTER  AND 
ELECTRICITY 


THE 

NATURE    OF    MATTER 

AND 

ELECTRICITY 

AN  OUTLINE   OF  MODERN  VIEWS 


BY 

DANIEL  F.  COMSTOCK,  S.B.,  Ph.D., 

ENGINEER  AND  ASSOCIATE  PROFESSOR  OF  THEORETICAL  PHYSICS 
IN  THE  MASSACHUSETTS  INSTITUTE  OF  TECHNOLOGY 

AND 

LEONARD  T.  TROLAND,  S.B.,  A.M.,  Ph.D., 

INSTRUCTOR  IN  HARVARD  UNIVERSITY 


ILLimPATED 


NEW  YORK 

D.   VAN   NOSTRAND    COMPANY 

25  PARK  PLACE 

1917 


COPYRIGHT,    1917 
BY  D.   VAN  NOSTRAND   COMPANY 


THE-PLIMPTON-PRESS 
NORWOOD-MASS-U'S'A 


PREFACE 

This  book  attempts  to  give  in  broad,  schematic  form 
the  conception  of  the  structure  of  the  material  universe 
which  has  developed  in  the  minds  of  modern  students 
of  physical  science.  The  treatment  of  the  subject 
which  is  here  offered  is  radically  elementary,  and  is 
intended  to  be  "popular"  if  not  "literary"  in  its 
style.  But,  although  elementary,  it  omits  none  of  the 
salient  general  ideas,  whether  these  belong  primarily 
to  the  sciences  of  chemistry,  electricity,  optics,  or  heat. 
It  is  characteristic  of  the  modern  standpoint  that  it 
permits  a  blending  of  all  of  the  physical  sciences 
into  a  single  world  view,  which  grows  in  unity  with 
the  years,  and  with  study.  A  glance  at  the  table  of 
contents  of  the  present  volume  will  reveal  what  may 
seem  to  the  uninitiated  reader  a  very  heterogeneous 
assemblage  of  topics,  but  it  is  the  hope  of  the  writers 
that  a  perusal  of  the  book  its  eh*  will  give  a  sense  of  the 
profound  inner  unity  of  all  of  these  outwardly  various 
matters. 

It  is  the  belief  of  the  authors  that  a  book  of  this  nature, 
written  in  the  light  of  the  most  recent  discoveries,  will 
find  a  welcome  amongst  the  scientific  laity,  as  well  as 
with  scientific  or  philosophic  workers  in  general  whose 
special  fields  are  perhaps  somewhat  removed  from  that 
of  theoretical  physics.  At  the  moment  of  writing  there 
is  no  book  available  dealing  with  the  whole  modern 
theory  of  matter  and  energy  in  either  an  elementary  or 
an  advanced  fashion,  and  treating  it  as  a  unit.  Many 

[v] 

3G7522 


PREFACE 

admirable  treatises  on  portions  of  the  field  are  of  course 
obtainable.  For  a  considerably  more  advanced,  yet 
not  very  difficult,  discussion  the  reader  is  referred  to 
three  books  which  together  cover  the  ground  fairly 
thoroughly,  o/z.,  The  sixth  edition  of  Nernst's  "Theoret- 
ical Chemistry,"  the  second  of  Campbell's  "Modem 
Electrical  Theory,"  and  Rutherford's  "Radio-active 
Substances  and  their  Radiations."  Specific  references 
to  other  works  are  given  at  the  end  of  each  Section  of 
Part  n  of  the  present  book. 

Something  must  be  said  in  explanation  of  the  arrange- 
ment of  the  book.  It  consists  of  two  parts,  the  first 
giving  a  rapid  survey  of  the  entire  subject,  outlining  the 
fundamental  conceptions  and  emphasizing  then*  most 
significant  applications  only,  while  the  second  retraces 
the  same  general  field  in  a  slower  and  less  connected 
way,  in  order  to  consider  details  omitted  hi  the  more 
cursory  treatment.  The  second  part  is  divided  into  fifty- 
six  sections,  each  of  which  is  numbered  and  referred  to 
by  its  number  in  the  appropriate  connection  in  Part  I. 

The  book  may  be  read  in  various  ways  according  to 
the  purposes  or  pleasure  of  the  reader.  If  he  is  interested 
only  to  acquaint  himself  with  the  fundamentals  of  the 
modern  theory  through  a  quick,  general  sketch  he  may 
read  Part  I  continuously  and  omit  Part  n  altogether. 
If,  on  the  other  hand,  he  is  already  familiar  with  these 
essentials  he  may  prefer  to  reverse  the  procedure  and 
omit  Part  I.  Part  II  although  definitely  divided  by  topics 
nevertheless  forms  a  fairly  continuous  discussion.  In 
general,  however,  the  best  method  of  using  the  book  will 
probably  be  to  read  both  of  the  sections  in  parallel,  refer- 
ring to  each  Section  in  Part  n  as  its  number  appears  in 
the  text  of  the  first  part.  It  is  believed  that  this  method 
of  study  will  encourage  the  type  of  attitude  which  is  re- 

[vi] 


PREFACE 

quired  to  give  the  subject  the  greatest  clearness  in  the 
reader's  mind.  It  is  obvious  that  the  structure  of  Part 
n  permits  its  ready  use  for  purposes  of  special  reference, 
such  as  may  arise,  for  example,  in  connection  with  school 
courses  in  elementary  physics  and  chemistry. 

The  basis  of  Part  I  is  to  be  found  in  a  series  of  articles 
contributed  in  1911  by  D.  F.  Comstock  to  the  "  Science 
Conspectus,"  the  journal  of  the  Massachusetts  Institute 
of  Technology  Society  of  Arts.  Somewhat  to  his  surprise, 
there  was  a  wide  demand  from  various  sources  for  fur- 
ther copies  of  these  articles,  and  hence  it  seemed  worth 
while  to  publish  them  in  book  form,  together  with  a  more 
complete  discussion  of  the  same  subject.  The  articles 
have  been  amplified  and  brought  up  to  date  by  their 
original  author. 

At  Professor  Comstock's  suggestion,  I  undertook  the 
writing  of  Part  II,  which  provides  the  more  elaborate 
treatment  just  mentioned. 

L.  T.  TROLAND 

Boston,  Mass. 


CONTENTS 


PART  I 

A  BRIEF  OUTLINE  OF  THE  MODERN  THEORY  OF 

MATTER,    ELECTRICITY   AND    ENERGY. 

(BY  D.  F.  COMSTOCK) 


I.  INTRODUCTORY 

II.  THE  ULTIMATE  REALITIES 


HI.  ATOMS  AND  THEIR  BEHAVIOR 2 

Their  Size;  Their  Shape;  The  Different  Kinds  of 
Atoms;  The  Tendency  Shown  by  Atoms  to  Form 
Groups;  Elements  and  Compounds;  Chemical  Ac- 
tion; Permanence  of  the  Atom;  General  Forces  of 
Attraction  Between  Atoms  and  Between  Groups  of 
Atoms. 

IV.  THE    NATURE    OF    HEAT    AND    ALLIED    PHE- 
NOMENA         11 

The  Motion  of  the  Molecules;  Molecules  are  Per- 
fectly Elastic;  Solid,  Liquid  and  Gas,  The  Causes 
of  Their  Differences;  The  Brownian  Movement  and 
the  Visibility  of  Heat  Motion ;  A  Model  of  a  Liquid ; 
A  Model  of  a  Solid;  How  Friction  Causes  Heat; 
Why  "  Evaporation  Cools  ";  The  "  Absolute  Zero  "; 
The  Heat  Energy  in  Bodies. 

V.  THE  ELECTRON  AND  ITS  BEHAVIOR 21 

Its  Size;  Its  Weight;  Its  Shape  and  Structure;  The 
Two  Electricities;  Both  Kinds  of  Electricity  Abun- 
dant hi  all  Bodies;  Electrons  Negatively  Charged; 
Atoms  and  Electricity;  Negative  Charge  means  "  Too 

el*: 


CONTENTS 

Many"  Electrons,  Positive  Charge  "Too  Few"; 
The  Electric  Current;  The  Action  of  a  Battery  or 
Dynamo;  "Free  Electrons";  The  "Evaporation" 
of  Electrons. 


VI.  ELECTRONS,  CHEMICAL  ACTION,  AND  LIGHT   .       27 
Electrons  and  Chemical  Action;  Electrons  and  Light; 
The  Absorption  of  Electric  Waves;    The  Reflection 
of  Electric  Waves;  The  Speed  of  Electric  Waves  in 
Different  Bodies. 

VH.  ELECTRONS  AND  MAGNETISM    ........       32 

The  Connection  of  Electricity  with  Magnetism;  The 
Deflection  of  Electrons  Caused  by  Magnetism;  The 
Action  of  a  Dynamo;  Permanent  Magnetism;  The 
Effect  of  Magnetism  on  Light. 


RADIO-ACTIVITY  ................       34 

The  Three  Rays;  The  Beta  Rays;  The  Alpha  Rays; 
The  Gamma  Rays;  The  Cause  of  Radio-  Activity; 
Successive  Disruptions  of  the  Atoms  of  Radio-Active 
Substances;  Radio-  Activity  not  a  Chemical  Change; 
Intra-  Atomic  Energy;  The  Quantity  of  Intra-  Atomic 
Energy;  The  Radio-Active  Elements;  Are  all  of  the 
Elements  Radio-Active?  The  Evolution  of  the  Ele- 
ments. 


IX.  THE  STRUCTURE  OF  THE  ATOM  .......         41 

General  Principles;  Evidence  for  Orderly  Structure 
in  the  Atom  ;  Spectral  Lines. 

X.  RECENT   DISCOVERIES   CONCERNING   ATOMIC 

STRUCTURE   AND   RADIATION  .......       43 

Recent  Advances  Concerning  the  Atom;  Atomic 
Numbers;  The  Quantum  Theory;  The  Similarity 
of  all  Forms  of  Radiant  Energy;  X  Rays. 

XL  ATOMS  AND  LIFE  .........   .   .....      60 


CONTENTS 

PART  II 

AN    APPENDIX    TO    PART    I,  CONSISTING    OF  FIFTY-SIX 

SECTIONS,    EACH     DISCUSSING     IN     FURTHER 

DETAIL     SOME     PROBLEM     MORE     BRIEFLY 

TREATED  IN  PART  I.     (BY   L.   T.   TROLAND) 


1.  THE   SOURCES   OF  THE   MODERN   THEORY   OF 

MATTER 62 

A  brief  statement  of  the  history  of  the  subject. 


2.  METHODS  OF  DETERMINING  ATOMIC  SIZES  .  .  63 
The  thickness  of  the  thinnest  known  films  of  matter; 
Calculations  based  on  the  volume  occupied  by  the 
atoms,  on  chemical  deposition  caused  by  the  electric 
current,  on  the  speed  of  ions,  heat  conduction,  etc.; 
Agreement  of  the  differently  obtained  results  amongst 
themselves. 


3.  ATOMS,  COLLOIDS  AND  THE  MICROSCOPE  ...       68 
Can  atoms  be  seen?    Nature  of  Colloids. 


4.  THE  SHAPE  OF  ATOMS 60 

Atoms  probably  spherical;    Means  of  showing  this; 
The  "  solar  system  "  idea  of  the  atom. 


6.  SPECIES   OF   ATOMS;    ATOMIC   WEIGHTS,   AND 

ATOMIC  VOLUMES 62 

Table  of  the  elements,  their  symbols,  atomic  weights, 
general  properties,  and  dates  of  discovery;  Table  of 
the  radio-active  atoms;  Methods  of  ascertaining  the 
relative  weights  of  atoms,  from  chemical  analysis,  from 
the  volumes  occupied  by  gases ;  How  the  volume  of  an 
atom  is  related  to  its  weight, 
[xi] 


CONTENTS 

6.  THE  PERIODIC  TABLE  OF  THE  ELEMENTS.    .    .       68 

Systematic  resemblances  between  different  elements; 
The  principle  of  the  Periodic  Table,  "families"  and 
"series"  of  elements;  Our  knowledge  of  elements  as 
yet  undiscovered;  Defects  in  the  Periodic  System; 
The  probable  meaning  of  the  system;  Prout's  Hypothe- 
sis: Helium  and  the  Nucleus  Theory;  Isotopes;  Meta- 
neon;  The  Table  itself. 

7.  THE   ARRANGEMENT   OF   THE   ATOMS   IN   THE 

MOLECULE 76 

The  multitudinous  compounds  of  carbon;  "  Isomers  " 
and  structural  formulae ;  Proof  that  our  conceptions  of 
molecular  structure  are  correct  in  the  case  of  "  ben- 
zene," as  an  example;  Molecules  of  single  elements; 
How  the  shape  of  crystals  depends  on  that  of  the  mole- 
cules composing  them. 

8.  THE    PHYSICAL    PROPERTIES    OF    COMPOUND 

SUBSTANCES 86 

What  determines  these  properties;  Meaning  of  color; 
Individuality  of  molecules ;  Allotropism ;  Recent  ideas 
concerning  the  basis  of  chemical  individuality. 

9.  CONCERNING   CHEMICAL  EQUATIONS 90 

The  types  of  chemical  change  and  the  way  in  which  the 
chemist  represents  them. 

10.  THE  FORCES  OF  ATTRACTION  WITHIN  BODIES  .       91 

Probable  relation  between  gravitation  and  the  attrac- 
tion between  individual  molecules  and  atoms;  De- 
pendency of  the  forces  of  cohesion,  etc.,  upon  those  of 
chemical  affinity,  and  of  the  latter  upon  the  forces  within 
the  atom  itself. 

11.  "THE    KINETIC    MOLECULAR    THEORY"     ...       92 

The  nature  of  this  theory  and  the  ideas  it  is  based  on ; 
The  idea  of  probability  and  the  use  of  averages  in  mo- 
lecular physics ;  Individuality  in  the  molecular  world. 

[xii] 


CONTENTS 

12.  THE  SPEEDS  OF  MOLECULAR  MOTION 94 

The  temperature  of  a  body  is  proportional  to  the  "  ki- 
netic energy "  of  its  molecules ;  Relative  speeds  of 
heavy  and  of  light  molecules  at  the  same  temperature ; 
The  actual  calculated  speeds  of  certain  molecules. 

13.  THE  AVERAGE  DISTANCE  TRAVERSED  BY  A  GAS 

MOLECULE   BETWEEN   IMPACTS 97 

Definition  of  "mean  free  path"  in  a  gas;  Properties 
of  a  gas  affected  by  size  of  this  path;  Its  length  about 
one  one-millionth  of  an  inch  under  ordinary  conditions. 

14.  DIFFUSION 99 

Its  cause  and  mechanism. 

16.   SOUND 101 

The  structure  of  a  sound-wave ;  How  it  is  set  up  and 
how  it  travels;  Similarity  between  sound-  and  heat- 
waves. 

16.  LATENT  HEATS 102 

Explanation  of  the  fact  that  heat  disappears  when  a 
body  melts  or  vaporizes.  Why  solids  soften  when 
heated;  Cause  of  the  "  surf  ace  tension"  of  liquids; 
The  mechanism  of  evaporation. 


17.  THE    "CRITICAL"    AND    BOILING    POINTS    OF 

LIQUIDS 106 

Definition  of  the  "critical  point"  of  a  liquid;  Change 
in  latent  heats  and  surface  tension  near  critical  point 
and  reason  therefor;  What  "boiling"  means  on  the 
molecular  theory. 

18.  THE   SIMPLE  LAWS   OF  GASES  AND   OF  SOLU- 

TIONS     106 

Why  the  pressure  exerted  by  a  gas  increases  with  its 

degree  of  confinement,  and  with  rise  in  temperature,  the 

laws  of  Boyle  and  of  Charles;   Absolute  Zero  and  the 

[xiii] 


CONTENTS 

principle  of  Gay-Lussac;  Explanation  of  the  law  of 
Avogadro ;  Effect  of  volume  of  the  molecules  and  their 
mutual  attractions  upon  the  laws  of  gases,  the  formula 
of  Van  Der  Waals. 


19.  OSMOTIC  PRESSURE 108 

Why  dissolved  substances  obey  the  same  general  laws 
as  gases. 


20.  HEAT  CONDUCTION 109 

The  cause  of  differences  in  the  heat  conductivity  of 
solids,  liquids  and  gases;  The  part  played  by  "free 
electrons  "  in  the  conduction  of  heat. 


21.  THE   BROWNIAN   MOVEMENT   AND    ITS    MEAS- 
UREMENT     110 

Method  of  studying  the  Brownian  movement;  Specific 
results  verifying  the  kinetic  molecular  theory. 


22.  THE  SOLID  AND  CRYSTALLINE   STATES 112 

The  difference  between  crystalline  and  "amorphous" 
bodies;  The  crystal  as  the  unit  of  structure  of  matter 
just  above  the  molecule ;  Crystal  structure  as  studied 
by  X  rays;  Liquid  crystals. 


23.  VAPOR   PRESSURE   AND   THE   LAW    OF  DISTRI- 
BUTION  OF  MOLECULAR   SPEEDS 115 

Although  for  a  given  temperature  all  of  the  molecules 
do  not  move  at  the  same  speed,  most  of  them  tend  to 
have  at  least  approximately  the  average  speed  for  all; 
How  this  fact  explains  the  manner  in  which  the  rapidity 
of  evaporation  of  liquid  increases  with  temperature; 
Similarly  with  respect  to  the  pressure  exerted  by  the 
resulting  vapor ;  Why  a  liquid  and  its  vapor  maintain  the 
same  temperature  in  spite  of  the  "cooling  effect  of 
evaporation." 


CONTENTS 

24.  HEAT  ENERGY  AND  SPECIFIC  HEATS 118 

Definition  of  the  "total  heat  energy"  of  a  body,  and  of 
"specific  heat";  Du  Long  and  Petit's  law  of  "atomic 
heats  "  and  its  explanation;  Explanation  of  the  constant 
relation  between  the  atomic  heats  of  solids  and  of  gases; 
Deviations  from  these  rules  and  their  probable  signifi- 
cance. 

25.  THE  DISCOVERY  AND  MEASUREMENT  OF  THE 

ELECTRON 120 

J.  J.  Thomson's  work  on  the  "cathode  rays";  How 
Thomson  determined  the  mass  and  charge  of  the  elec- 
tron; Counting  electrons  by  the  use  of  a  fog;  How  the 
size  of  the  electron  can  be  calculated;  Its  substance 
and  its  structure. 

26.  THE  IMPORTANCE  OF  ELECTRICAL  FORCES  IN 

NATURE 126 

All  physical  events  probably  determined  by  such  forces 
in  the  last  analysis. 

27.  THE  REACTIONS  OF  ELECTRONS  AND  CHARGED 

ATOMS 125 

Definition  of  an  "ion,"  and  how  ions  are  produced; 
Energy  required  to  drag  an  electron  from  an  atom; 
How  electrons  and  ions  of  different  kinds  act  on  one 
another;  Rules  for  such  action. 

28.  SOME  EFFECTS  CONNECTED  WITH  THE  ELEC- 

TRICAL CURRENT 129 

The  "Hall  Effect,"  why  magnetism  deflects  an  electric 
current;  Nature  of  electrical  "resistance";  Signifi- 
cance of  "amperage " ;  Why  the  best  electrical  conduc- 
tors are  also  the  best  heat  conductors,  and  why  metals 
are  in  general  superior  to  other  substances  hi  these 
respects ;  The  motion  of  electrons  in  a  wire  is  opposite 
in  direction  to  the  "current." 

29.  ELECTRICAL      CONDUCTION     IN      GASES     AND 

LIQUIDS 131 

Ions  carry  electricity  in  these  substances;  Nature  of 
electro-chemical  action,  or  "  electrolysis." 


CONTENTS 

30.  THE  ELECTRICAL  TRANSMISSION  OF  POWER.    .     133 

Mechanism  of  this  transmission. 

81.  THERMO-ELECTRICITY 133 

The  various  "affinities"  of  different  substances  for 
electrons;  the  operation  of  a  "thermopile"  explained 
on  the  electron  theory;  The  elements  arranged  in  order 
of  their  affinities  for  electrons. 

32.  CHEMICAL  AFFINITY 136 

Electro-negative  and  electro-positive  elements;  Ions 
and  electrons  in  chemical  action;  Electro-negativity  or 
positivity  only  a  relative  conception;  How  atoms  of  the 
same  species  can  be  attracted  electrically;  Nature  of 
chemically  "inert"  elements. 

33.  SOLUTION  AND   ELECTRICAL  DECOMPOSITION.     139 

How  water  can  "ionize"  substances  which  dissolve  in 
it;  Definition  of  "electrolytic  dissociation";  Motion  of 
the  ions  in  a  solution  under  the  influence  of  electrical 
force;  How  it  is  proven  that  water  dissociates  dis- 
solved substances,  effect  on  boiling  and  freezing 
points. 

34.  CHEMICAL  VALENCY 141 

Definition  and  cause  of  valency. 

35.  CHEMICAL  ACTION 142 

The  complexity  of  the  changes  involved  in  chemical 
action;  Chemical  change  depends  on  the  chance  col- 
lision of  molecules;  Explanation  of  the  fundamental 
"law  of  chemical  mass  action"  on  this  basis;  Rever- 
sible and  irreversible  chemical  processes;  Chemical 
equilibrium  and  its  kinetic  nature. 

36.  EFFECTS     AND     CONDITIONS     OF     CHEMICAL 

CHANGE 144 

Heat  and  chemical  change;  How  electric  current  and 
light  can  be  generated  by  chemical  action;  Nature  of 
"chemical  energy." 


CONTENTS 

37.  LIGHT    WAVES    AND    LINES    OF    ELECTRICAL 

FORCE 146 

Present  status  of  the  "aether"  theory;  Definition  and 
nature  of  a  line  of  electrical  force;  Formation  of 
"kinks"  in  such  lines;  Light  not  a  continuous  wave- 
motion. 

38.  THE   ZEEMAN   EFFECT 149 

General  nature  of  the  theory  of  the  effect,  and  results 
of  its  application  to  the  phenomena;  The  Stark  Effect. 

39.  THE    CONDITIONS    UNDER    WHICH    LIGHT    IS 

PRODUCED 150 

Temperature  radiation;  Why  the  light  from  a  glowing 
body  is  whiter  the  hotter  the  body;  The  law  connect- 
ing wave-length  and  energy  of  emitted  light  with  tem- 
perature, and  its  general  explanation  in  terms  of  the 
electron  theory ;  The  emission  of  light  by  gases ;  loni- 
zation  and  the  production  of  "line  spectra";  Spectral 
"series." 

40.  THE  GAMUT  OF  ELECTRICAL  WAVES 155 

The  complete  spectrum,  including  all  electrical  waves; 
Position  of  visible  light,  "ultra-violet,"  "infra-red," 
heat,  "Hertz  waves,"  X  rays,  etc.,  in  this  spectrum; 
Velocity  of  light;  Actual  lengths  and  frequencies  of 
light  and  other  electrical  waves. 

41.  COLOR    AND    THE    ABSORPTION    AND    REFLEC- 

TION OF  LIGHT 157 

How  color  is  produced  by  absorption;  Explanation  of 
the  "selective  absorption"  of  light;  Basis  of  the  sen- 
sations of  color;  Production  of  color  by  reflection. 

42.  THE   REFRACTION    OF   LIGHT 159 

How  a  column  of  light  is  bent  in  passing  from  air  into 
glass;  Definition  of  "dispersion"  and  statement  of 
the  law  governing  it;  Relation  between  the  index  of 
refraction  of  a  substance  and  its  "dielectric  capacity." 

[xvii] 


CONTENTS 

43.  ROWLAND'S   EXPERIMENT 161 

How  it  was  shown  that  the  motion  of  an  electrical  charge 
causes  magnetism. 

44.  THE  DEFLECTION  OF  MOVING  ELECTRONS  BY 

A  MAGNET 162 

How  the  experiment  is  performed. 

46.  ALL    BODIES    ARE    MAGNETIC 163 

The  two  kinds  of  magnetism;  How  permanent  magnet- 
ism is  possible. 

46.  THE   RADIO-ACTIVE    SUBSTANCES 165 

The  Work  of  Becquerel  and  the  Curies;  The  "Radium 
series";  The  law  of  decay  of  radio-active  substances; 
Their  position  in  the  Periodic  Table. 

47.  HOW  THE  RAYS  FROM  RADIUM  ARE  STUDIED  .     168 

The  differential  effect  of  magnetism  on  the  rays ;  Pene- 
trating power  of  the  beta  rays. 

48.  HOW  RUTHERFORD  PROVED  THE  ALPHA  RAYS 

TO   BE   HELIUM   ATOMS 169 

Description  of  the  experiment. 

49.  THE  NATURE  OF  THE   GAMMA  RAYS 170 

Relation  of  the  gamma  rays  to  the  beta  rays  and  the 
disruption  of  the  radio-active  atom ;  Secondary  gamma 
rays. 

60.  THE  ENERGY  OF  THE  ATOM 172 

The  great  stability  of  the  atom;  Relation  of  intra-  to 
inter-atomic  forces  and  energies. 

51.  THE  RADIO-ACTIVITY  OF  POTASSIUM 173 

The  work  of  Campbell. 

[xviii] 


CONTENTS 

62.  INORGANIC  EVOLUTION 173 

The  variability  of  the  line  spectra  of  the  elements;  The 
spectra  shown  by  the  hottest  stars  are  the  most  imper- 
fect; Lockyer  has  shown  that  the  very  hottest  stars  con- 
tain only  the  simplest  elements;  Meaning  of  these 
facts. 

53.  THEORIES  OF  THE  STRUCTURE  OF  THE  ATOM  .     174 

Thomson's  theory  and  its  partial  explanation  of  the 
mystery  of  the  Periodic  Table;  The  modern  "Nucleus 
Theory  " ;  The  empirical  basis  of  this  latter  theory ;  The 
number  of  electrons  in  the  atom;  Isotropism;  Atomic 
numbers ;  The  hydrogen  atom,  its  constitution  and  the 
basis  of  its  line  spectrum  as  deduced  from  the  "  Quan- 
tum Theory  "  of  light. 

54.  THE  QUANTUM  THEORY  OF  RADIANT  ENERGY  .     182 

The  nature  of  the  theory;  The  photo-electric  effect; 
The  relation  between  the  "frequency"  and  energy  of 
light  quanta;  The  conditions  of  the  absorption  and  emis- 
sion of  quanta;  The  explanation  of  the  low  values  of 
specific  heats  near  absolute  zero  temperature ;  Planck's 
original  reason  for  propounding  the  theory;  The  broad 
significance  of  the  theory;  The  doctrine  of  entropy  and 
its  basis  in  the  theory  of  probabilities;  Entropy  and 
radiation. 

65.  X  RAYS   AND   THEIR   MEASUREMENT 189 

The  origin  and  nature  of  X  rays;  Characteristic  X  rays; 
Why  Xrays  penetrate  "opaque"  bodies;  The  cor- 
puscular properties  of  X  rays;  The  reflection  and 
"diffraction"  of  X  rays  by  crystals;  New  light  on 
crystal  structure. 

66.  LIFE  AND   CATALYSIS 193 

Vital  phenomena  are  consistent  with  an  explanation  in 
terms  of  atoms,  molecules  and  electrons. 

INDEX 196 

[xix] 


ILLUSTRATIONS 

Page 

Sir  Joseph  J.  Thomson Frontispiece 

Fig.    1.   The  Relative  Sizes  of  Atoms  and  Molecules    ...  3 

Fig.    2.  Relative  size  of  Molecules  and  Visible  Particles    .  4 

Fig.    3.  Water  Molecules Facing  page  4 

Fig.    4.  Molecules  of  Steam 6 

Fig.    5.  Atoms  of  a  Liquid 6 

Fig.    6a.  Formulae  of  some  Common  Organic  Compounds  .    .  8 

Fig.    6b.  Formula  of  an  Azo-dye 9 

Fig.    7.  Atoms  of  a  Solid Facing  page  10 

Fig.    8.   Gas  Molecules 12 

Fig.    9.  Vapor  Molecules  at  the  Surface  of  a  Liquid    ....  15 

Fig.  10.  The  Constitution  of  a  Simple  Molecule 28 

Figs,  lla,  b.  The  Radio-Active  Elements  and  their  Relation- 
ships and  Rays 36,  37 

Fig.  12.   Five  Isomeric  Hydrocarbons  having  the  Constitution 

C6H14 79 

Figs.  13a,  b.  Benzene  and  its  Chlorine  Derivatives  ....    81,  82 

Fig.  14.   Models  of  Tartaric  Acid  Molecules 84 

Fig.  15.  Crystals  of  "Right"  and  "Left"  Tartaric  Acids    .    .  86 

Fig.  16.  Diffusion  Paths 100 

Fig.  17.  "Distribution  Curve"  for  Molecular  Speeds   ...  116 
Fig.  18.  Vacuum  Tube  to  show  the  Action  of  the  Cathode 

Rays 121 

Fig.  19.  How  the  Cathode  Rays  May  be  Bent  by  a  Magnet     .  122 

Fig.  20.  The  Forces  Acting  Between  Ions,  Atoms  and  Electrons  127 

Fig.  21.  A  Thermo-Electric  Circuit 134 

Fig.  22.  Showing  the  Manner  in  which  Two  Neutral  Aggre- 
gates of  Electrical  Particles  may  attract  eath  other.  138 
Fig.  23.  To  Show  how  Radiation  is  Produced  by  Stopping  the 

Motion  of  an  Electrical  Particle 148 


ILLUSTRATIONS 

Fig.  24.  The  Zeeman  Effect 160 

Fig.  26.  Curve  Showing  the  Relative  Intensities  of  Radiation 
of  Different  Wave  Lengths  Emitted  by  Solid  Bodies 
at  Various  Temperatures 153 

Fig.  26.  The  Direction  of  the  Magnetic  Forces  about  a  Moving 

Electrical  Charge  162 

Fig.  27.   Structural  Plan  of  a  Simple  Crystal 192 

PLATES 

I.  Line  Spectrum  of  Iron Facing  page  42 

n.   Cavendish  Gravitation  Apparatus     ....  "        "  92 

in.  Thomson  Cathode  Ray  Tube "        "  122 

IV.  The  Deflection  of  Cathode  Rays  by  a  Magnet.  "        "  162 

V.   Coolidge  X  Ray  Tube "       "  190 


[xxii] 


PART  I 

A  BRIEF  OUTLINE  OF  THE  MODERN  THEORY 
OF  MATTER,  ELECTRICITY,  AND    ENERGY 

CHAPTER  I 
INTRODUCTORY 

During  the  last  two  decades  there  has  been  a  very 
great  advance  in  our  knowledge  of  the  ultimate  constitu- 
tion of  matter.  (1)  The  older  ideas  which  prompted  the 
contemptuous  phrase  " gross  matter"  are  inadequate 
to  represent  the  extraordinary  complexity  and  delicacy 
of  structure  which  have  since  been  revealed.  The  end 
is  of  course  not  yet,  but  throughout  all  this  advance 
there  has  been  singularly  little  in  former  ideas  which 
had  to  be  considered  totally  wrong.  They  were  right  as 
far  as  they  went,  although  insufficient,  and  so  it  proba- 
bly is  with  our  present  ideas  respecting  the  structure  of 
things;  they  will  doubtless  appear  crude  in  the  light  of 
future  knowledge  but  in  a  general  way  they  are  proba- 
bly right  as  far  as  they  go,  and  hence  are  worthy  of  our 
attention. 

CHAPTER    H 

THE  ULTIMATE  REALITIES 

According  to  the  modern  theory  of  matter  all  bodies 
are  complex  structures  composed  of  small  particles  called 

NOTE:  The  full-face  numbers  inserted  in  the  text  at  various 
points  refer  to  the  Sections  of  Part  II  in  which  related  subjects  are 
discussed  (see  Preface),  or  in  which  further  details  are  given  on 
the  same  subject. 

[1] 


ATOMS  AND  THEIR  BEHAVIOR      [Chap.  IE 

atoms,  together  with  still  smaller  particles  known  as 
electrons.  If,  therefore,  we  were  familiar  with  the  laws 
of  action  of  atoms  and  electrons  we  should  understand 
completely  all  the  physical  phenomena  in  nature.  The 
atom,  as  we  shall  see  later,  is  a  much  more  complex 
structure  than  the  electron,  so  that  atoms  and  elec- 
trons are  not  quite  on  a  par  as  regards  classification, 
except  from  the  introductory  point  of  view,  from  which 
we  begin  discussion. 

As  a  third  fundamental  entity,  there  should  be  men- 
tioned the  energy  associated  with  atoms  and  electrons, 
but  for  the  present  this  will  be  left  out  of  consideration. 


CHAPTER    HI 

ATOMS  AND  THEIR  BEHAVIOR 

Size.  —  Atoms  are  minute  particles  each  about  one 
three-hundred-millionth  of  an  inch  in  diameter  (2).  If 
the  earth  were  made  up  of  base-balls  it  would  be  a  fair 
model  of  a  drop  of  water  made  up  of  atoms.  The  most 
powerful  microscope  known,  used  under  the  best  condi- 
tions, would  enable  us  to  see  an  object  approximately 
two  hundred  atoms  in  width.  Single  atoms  are,  there- 
fore, totally  invisible,  and  their  properties  cannot  be 
found  out  by  direct  inspection  (3). 

Shape.  —  Not  much  is  known  as  regards  the  shape 
of  the  atoms,  but  in  general  they  behave  as  if  they  were 
not  very  far  from  spherical  (4). 

Different  Kinds. — We  are  now  acquainted  with  about 
one  hundred  different  kinds  of  atoms,  that  is,  different 
species.  The  individual  atoms  in  each  species  are,  how- 
ever, exactly  alike,  or  have  so  nearly  the  same  properties 
that  under  most  conditions  there  is  no  difference  in  the 

[2] 


Chap.  HI]  SPECIES  OF  ATOMS 

action  of  the  individuals.    Atoms  of  different  kinds  differ 
in  size  and  still  more  in  weight. 

At  present  there  is  no  agreement  as  to  the  difference 
in  size  of  the  various  kinds  of  atoms.  On  the  basis  of 
certain  calculations  from  coefficients  of  expansion,  some 


Fig.  1 
THE  RELATIVE  SIZES  OF  ATOMS  AND   MOLECULES 

This  diagram  is  intended  to  give  an  idea  of  the  relative  magnitudes 
of  atoms  and  molecules.  However,  the  drawings  are  only  symbolic,  as 
the  dimensions  have  been  calculated  on  the  assumption  that  the  mole- 
cules are  spherical,  which  cannot  be  strictly  true.  It  will  be  noticed  that 
the  smallest  atom  (that  of  hydrogen)  differs  only  slightly  in  size  from 
the  largest  atom  (that  of  uranium).  The  starch  molecule  is  probably 
one  of  the  largest  which  exists  and  it  will  be  seen  that,  according  to  the 
diagram,  it  is  very  much  larger  than  the  largest  atom  or  than  the  mole- 
cule of  sugar.  The  relative  weights  of  the  particles  represented  are  as  fol- 
lows: Hydrogen,  1;  Uranium,  239;  Sugar,  366;  and  Starch,  not  accu- 
rately known  but  probably  about  25,000.  A  molecule  of  ordinary  alcohol 
weighing  46,  would  be  slightly  larger  than  the  uranium  atom. 

investigators  believe  that  the  sizes  of  the  different  kinds 
of  atoms  are  in  the  same  order  as  their  weights.  Ac- 
cording to  this  view,  therefore,  the  lightest  atom,  hydro- 
gen, is  also  the  smallest;  and  the  heaviest  atom,  uranium, 
is  also  the  largest.  The  atom  of  uranium  is  about  240 
times  as  heavy  as  the  atom  of  hydrogen,  whereas  it  has 

[3] 


ATOMS  AND  THEIR  BEHAVIOR     [Chap.  HI 

only  about  two  and  one-half  times  as  great  a  diameter. 
If  this  view  is  correct,  we  might  represent  an  atom  of 
hydrogen  by  a  wooden  ball  the  size  of  a  pea,  and  an 
atom  of  uranium  by  a  lead  ball  the  size  of  a  cherry. 
Representatives  of  all  of  the  other  atomic  species  would 
then  be  arranged  in  a  complete  series  from  the  small 
wooden  ball  to  the  large  lead  one. 


Fig.  2 

RELATIVE  SIZE  OF  MOLECULES  AND  VISIBLE  PARTICLES 
The  molecule  represented  in  this  diagram  is  the  starch  molecule  of 
Figure  1,  very  much  reduced  in  scale.  It  is  not  certain  that  the  starch 
molecule  is  the  largest  which  exists,  but  it  is  very  far  from  being  the 
smallest.  The  microscopic  particle  which  is  represented  is  the  small- 
est which  can  actually  be  seen  under  the  most  powerful  microscope. 
Particles  nearly  as  small  as  the  starch  molecule  can  be  seen  indirectly 
by  means  of  the  ultra-microscope.  (See  Section  3.) 

Although  the  individual  atoms  of  one  kind  are,  with 
certain  modern  reservations,  all  alike,  those  of  different 
species  have  decidedly  different  properties,  and  this 
difference  hi  property  is  what  gives  variety  to  the  phys- 
ical world  as  we  see  it.  The  atoms  of  one  species  are 

[4] 


Chap.  IH]  MOLECULES 

so  definite,  unique,  and  characteristic  in  their  actions 
and  properties  that  they  give  one  the  impression  of  a 
delicacy  and  complexity  of  structure  suggestive,  almost, 
of  the  complexity  of  personality.  There  are  subtle  resem- 
blances between  one  species  and  another  with  regard  to 
one  -property,  and  marked  differences  with  regard  to 
another  (6).  Hence  one  should  be  very  careful  to  realize 
that  when,  for  reasons  of  analogy,  we  represent  an  atom 
as  a  ball  of  wood  or  a  ball  of  lead  we  are  representing  it 
only  in  the  vaguest  general  way,  and  are  totally  ignoring 
its  complexity  and  individuality. 

Tendency  to  Form  Groups  (Molecules).  —  Atoms  tend  to 
form  groups  known  as  molecules.  The  atoms  in  a  mole- 
cule adhere  with  considerable  force  and  some  molecules 
can  be  broken  up  only  with  the  greatest  difficulty.  These 
groups  have  a  definite  individuality  and  unless  acted  on 
from  the  outside  they  are  apparently  permanent.  The 
same  atoms  may  be  grouped  into  quite  different  molecules 
just  as  the  same  bricks  may  be  used  to  build  a  church 
or  a  jail,  or  the  same  letters  used  to  form  altogether 
different  words.  The  individuality  of  a  molecule  is  per- 
haps best  appreciated  by  thinking  of  the  individuality 
of  a  word.  A  word,  though  consisting  solely  of  letters, 
has  a  definite  unity  of  its  own.  A  molecule  made  up  of 
atoms  is  just  as  definite  an  aggregate  (7). 

As  a  help  toward  a  concrete  conception  two  drawings 
are  given  in  Figures  3  and  4.  The  first  represents  sym- 
bolically water  molecules,  each  consisting  of  two  hydro- 
gen atoms  and  one  oxygen  atom,  in  the  closely  crowded 
state  known  as  liquid,  and  the  second  the  same  mole- 
cules in  the  more  dispersed  state  known  as  vapor 
(steam). 

Elements  and  Compounds.  —  When  the  atoms  making 
up  the  molecules  of  a  substance  are  all  alike,  that  is  when 

[6] 


ATOMS  AND  THEIR  BEHAVIOR     [Chap,  m 

they  belong  to  the  same  species,  the  substance  is  called 
an  "element."  An  element,  therefore,  is  composed  of 
only  one  kind  of  atom.  A  compound  is  a  substance  the 
molecules  of  which  are  made  up  of  more  than  one  kind 
of  atom. 

Figure  5  represents  symbolically  a  liquid  element, 
Figure  3  a  liquid  compound.  "Oxygen,"  "hydrogen," 
"carbon,"  "lead,"  "copper,"  are  names  of  some  of  the 
elements,  and  they  are,  therefore,  names  of  atomic  species. 
"Water,"  "salt,"  "sugar,"  and  "carbon-dioxide"  are 
names  of  compounds,  and  hence  are  the  names  of 
molecular  species. 

The  water  molecules  in  Figure  3  are  seen  to  consist  of 
one  large  atom  and  two  smaller  ones  in  a  group.  The 
large  atom  is  an  oxygen  atom.  The  two  smaller  ones  are 
hydrogen  atoms  and  the  group  as  a  whole  is  a  water 
molecule.  It  is  therefore  true  to  say  that  a  water  mole- 
cule is  the  smallest  particle  of  water  that  it  is  possible  to 
have,  for  if  it  is  further  broken  up  it  is  no  longer 
water.  These  drawings  are  in  no  sense  other  than 
symbolic. 

As  a  moment's  thought  will  show,  thousands  upon 
thousands  of  different  kinds  of  molecules  are  known. 
Some,  such  as  the  water  molecule,  are  relatively  simple 
and  composed  of  a  few  atoms,  and  some,  such  as  the  sugar 
molecule,  are  very  complex.  (See  Figure  6.)  In  the  par- 
ticular case  of  the  sugar  molecule  the  number  of  atoms 
is  forty-five. 

A  small  crystal  of  "granulated"  sugar  is,  therefore,  a 
solid  mass  consisting  of  hundreds  of  millions  of  sugar 
molecules,  that  is  hundreds  of  millions  of  definite,  co- 
herent groups  of  atoms,  each  group  containing  twelve 
atoms  of  carbon,  twenty-two  of  hydrogen,  and  eleven  of 
oxygen. 

[63 


Chap.  IH]  CHEMICAL  ACTION 

The  definiteness  of  molecular  structure  must  not  be 
forgotten.  One  of  these  sugar  molecules  might  be  com- 
pared to  a  word  of  forty-five  letters,  for  if  a  single  atom 
were  removed  from  the  group,  or  if  a  single  atom  had  its 
position  markedly  changed,  the  group  might  still  be  a 
molecule,  but  it  would  not  be  a  molecule  of  sugar,  and  a 
vast  mass  of  such  modified  molecules  would  not  make 
up  a  crystal  having  the  same  properties  as  the  one  with 
which  we  started  (8). 

Chemical  Action.  —  Chemical  action  is  the  name  given 
to  the  process  in  which  the  groups  known  as  molecules 
are  either  formed  or  destroyed.  When  a  substance  is 
burned  or  when  an  acid  "eats"  a  metal  the  action  in- 
volves the  formation  of  new  molecules,  because  of  the  re- 
arrangement of  the  atoms,  and  therefore  the  production 
of  new  substances.  By  an  inspection  of  Figure  3  it  will 
be  clear  that  if  two  of  the  "large "  oxygen  atoms  could  be 
separated  from  their  respective  water  molecules,  to  re- 
combine  as  represented  in  Figure  8,  there  would  be  left 
behind,  four  of  the  "small"  hydrogen  atoms  of  Figure  3 
(two  from  each  decomposed  molecule),  and  these  would 
adhere  in  twos  and  would  form  two  hydrogen  molecules. 
When  thousands  of  molecules  were  thus  broken  up  the 
complete  process  would  be  called  the  decomposition  of 
water  into  oxygen  and  hydrogen.  It  is  easily  accom- 
plished in  the  laboratory. 

It  is  clear  that  when  complex  molecules  are  present  it 
is  possible  for  the  rearrangement  which  takes  place  to  be 
very  complicated  indeed.  There  are  often,  also,  several 
possibilities  of  rearrangement,  each  resulting  in  a  dif- 
ferent set  of  substances  as  a  final  outcome. 

Outside  conditions  such  as  temperature,  pressure,  etc., 
have  marked  effects  on  the  results  of  chemical  reactions. 

Chemical  action,  therefore,  always  implies  the  break- 

[7] 


ATOMS  AND  THEIR  BEHAVIOR     [Chap.  HI 


H    H 
Ethyl  Alcohol      H—  C—  C—  O— H 


H 
Acetic  Acid         H—  C—  C=  O 


O-H 


H 

H-C-O-H 
H— O— C— H 

Grape  Sugar       H— O—  C— H 
(dextrose)  H— C— O— H 

H— O— C-H 
0=C— H 
Fig.  6a 

FORMULA  OF  SOME  COMMON   ORGANIC  COMPOUNDS 

NOTE.  —  The  formula  of  ordinary  cane  sugar  (or  saccharose)  is  not  definitely 
established,  but  probably  consists  of  two  groups  of  atoms  similar  to  that  for 
dextrose,  combined  with  a  molecule  of  water. 


[8] 


Chap.  IIT|  COMPLEX  MOLECULES 

O 
H  H  H    H  H    H 

c     c  U        U 

/.   \  /  .\  S        \        S        \ 

H-C  C  C-N=N-C  C-C  C-N= 

c      c  xc=/      xc=cx 

Y  A  A        A  A 

H     O=S=O 

i 

Na 

H 

H— C— H 

H  H  H 

U         A    A 

=K-/          Vp-f-/    V    %C-H 
C=CX  H-C  C  C-H 

A 


o=s=o 

o        c-c  I 

Na—  O—  S  —  C  C—  O— H 

\  /  Na 

O  C=  C 


H—  C  C— H 

C=C 

HH 


Fig.  6b 
FORMULA  OF  AN  AZO-DYE 


ATOMS  AND  THEIR  BEHAVIOR     [Chap.  HI 

ing  up  or  forming  of  molecules,  and  in  general  it  means 
both  (9). 

Permanence  of  the  Atom.  —  Although  every  one  of  the 
thousands  of  "chemical  reactions"  which  are  daily  going 
on  hi  the  world  involves  the  formation  or  decomposition 
of  molecules,  no  way  has  yet  been  found  to  change  an  atom 
of  one  species  into  one  of  another  species.  To  find  such 
a  way  was  the  hope  of  the  alchemists  but  it  was  never 
realized.  We  shall  see  later  that  the  atoms  are  probably 
not  absolutely  permanent  but  that  the  mysterious  forces 
which  preserve  their  integrity  are  so  much  greater  than 
the  forces  which  hold  them  together  in  molecules,  that 
as  yet  it  has  not  been  found  possible  to  shatter  them 
artificially. 

The  atoms  have  great  family  attraction  and  it  is  the 
business  of  the  chemist  to  make  use  of  this  attraction  in 
the  service  of  man,  but  the  instinct,  we  might  say,  of  self- 
preservation  is  so  vastly  greater  hi  the  atom  than  its 
group-forming  tendency,  that  although  it  submits  to  the 
breaking  of  family  ties  it  will  not  allow  its  own  individual- 
ity to  be  tampered  with. 

To  state  that  it  will  never  be  possible  to  break  up  atoms 
artificially  would  of  course  be  folly,  but  we  can  say  at 
present  with  considerable  certainty  that  the  disruption 
of  atoms  must  involve  forces  of  an  entirely  different 
magnitude  from  those  called  "  chemical,"  which  are 
associated  with  the  breaking  up  of  molecules  into 
atoms. 

We  shall  see  later  that  there  appears  to  be  going  on  in 
nature  a  spontaneous  decomposition  of  the  atoms  of 
radium  and  certain  other  substances,  but  thus  far  it  has 
been  found  quite  impossible  artificially  to  influence  this 
spontaneous  disruption  to  the  slightest  degree,  and  there- 
fore although  we  might  call  the  process  "  natural  al- 

[10] 


. 

-i^.  ^aafc       lip        -,        III 

••*•** 


• 
' 


• 


Chap.  IV]  MOLECULAR  MOTION 

chemy"  it  cannot  be  called  alchemy  in  the  ordinary 
sense  of  the  word. 

General  Forces  of  Attraction.  —  Just  as  atoms  have 
forces  of  attraction  which  hold  them  together  in  mole- 
cules, so  molecules  attract  each  other  and  tend  to  form 
the  large  aggregates  which  we  call  "  objects."  These 
forces  are  in  general  weaker  than  the  forces  between  the 
atoms  (10).  They  are  great  enough,  however,  to  account 
for  the  relatively  strong  cohesion  of  solid  bodies  and  the 
weaker  cohesion  of  liquids. 


CHAPTER   IV 

THE  NATURE  OF  HEAT  AND  ALLIED   PHENOMENA 

The  Motion  of  the  Molecules.  —  It  is  now  believed,  and 
the  odds  amount  almost  to  certainty  (11),  that  all  atoms 
of  all  substances  are  in  ceaseless  motion  to  and  fro. 
This  motion  is  what  we  call  the  heat  of  a  body.1  The 
more  violent  the  motion  the  hotter  the  body  (12).  It  is 
perhaps  a  pity  that  the  motion  picture  art  is  not  de- 
veloped to  a  point  which  would  enable  us  to  embody 
this  violent  vibration  in  the  accompanying  figures,  so 
we  must  request  imagination  to  aid  incompetent  art 
and  to  endow  every  atom  or  molecule  of  Figures  3  to  8 
with  a  rapid  motion;  a  motion  which,  like  the  modern 
idea  of  freedom,  is  limited  only  by  the  equal  rights  of 
all  the  other  atoms. 

Thus  in  the  liquid  and  solid  states  (Figures  3,  5  and  7) 
the  crowding  is  so  close  that  "the  rights  of  others" 

1  Strictly  speaking,  it  is  the  energy  of  the  atomic  or  molecular 
motion  which  constitutes  the  heat  of  a  body.  The  distinction 
between  atoms,  molecules  and  electrons  is  not  important  when 
heat  phenomena  are  being  described  in  a  general  way  since  it  is 
probable  that  all  of  the  particles  share  in  the  motion. 

[11] 


HEAT  PHENOMENA  [Chap.  IV 

allow  only  vibration  through  a  very  limited  distance, 
while  in  the  gaseous  state  (Figures  4  and  8),  the  motion 
consists  of  a  straight  line  flight  until  by  chance  there  is 
an  encounter  with  another  molecule.  When  this  occurs 
there  is  a  rebound  and  then  another  flight.  The  distance 
between  the  molecules  is  so  small  and  their  speed  is  so 
large  that  literally  billions  of  these  impacts  are  occurring 
every  second  in  even  a  cubic  inch  of  gas  (13). 

Molecules  Have  No  Friction.  —  It  is  necessary  in  order 
that  this  motion  should  continue  indefinitely  that  the 
atoms  or  molecules l  be  considered  frictionless.  At  first 
sight  this  seems  an  improbable  hypothesis,  but  when  the 
nature  of  friction  is  understood  from  the  present  point  of 
view  the  assumption  is  seen  to  be  justified.  A  number  of 
billiard  balls  put  on  a  table  and  set  going  would  bound  to 
and  fro  for  a  short  time  and  gradually  come  to  a  stop,  but 
this  is  because  at  every  impact  part  of  the  energy  of  the 
balls'  motion  is  wasted  in  the  form  of  heat,  that  is,  the 
balls  are  actually  a  trifle  warmer  after  striking  each  other 
than  before.  Now  from  the  present  point  of  view,  as  we 
have  just  seen,  "warmer"  means  more  rapid  molecular 
motion,  so  that  the  billiard  balls  gradually  slow  down 
and  stop  because  their  motion  is  gradually  transformed  into 
the  invisible  motion  of  the  molecules  which  compose  them 
and  surrounding  objects.  Thus  friction  exists  between 
visible  objects  solely  because  of  the  existence  of  the 
molecules  which  go  to  make  them  up,  these  molecules 
absorbing  the  motion. 

From  this  it  is  clear  that  for  two  molecules  to  waste 
energy  in  impact  after  the  manner  of  two  billiard  balls, 

1  The  argument  here  presented  applies  strictly  only  to  the 
"  ultimate  particles  "  of  matter,  whatever  these  may  be.  How- 
ever, no  very  important  inaccuracy  can  result  from  identifying 
these  ultimate  particles  with  atoms,  molecules  and  electrons. 

[12] 


I 


I  f 


Chap.  IV]         MOLECULES  AND  FRICTION 

it  would  be  necessary  for  them  to  be  composed  of  still 
smaller  "molecules."  Hence,  the  ultimate  particles 
themselves  cannot  possess  friction  in  the  ordinary  sense. 

As  a  matter  of  fact  we  shall  see  later  that  molecules 
do  lose  energy,  not  in  a  way  similar  to  the  friction  of  large 
objects,  but  by  radiating  it  in  the  form  of  heat  waves.  An 
analogy  for  this  radiation  of  energy  may  still  be  found 
among  billiard  balls,  for  if  they,  the  table,  the  cushions 
against  which  they  strike,  and  the  air  around,  were  all 
perfectly  elastic  and  frictionless,  the  balls  when  set  going 
would  keep  then*  motion  for  a  far  greater  time  than  they 
actually  do,  but  they  would  not  continue  to  move  indefi- 
nitely. This  is  because  at  each  impact  between  two  balls 
there  would  still  be  a  sound,  the  "crack"  with  which 
we  are  all  familiar,  and  this  would  carry  energy  away. 
This  loss  of  energy  by  sound  is  vaguely  similar  to  the 
radiation  of  heat  energy  by  the  molecules  of  a  substance 
(15). 

Solid,  Liquid  and  Gas.  —  All  substances  which  do  not 
decompose  (that  is,  all  substances  the  molecules  of  which 
do  not  break  up)  on  heating,  are  capable  of  existing  in 
three  states,  the  solid,  the  liquid  and  the  gaseous.  A 
solid  when  heated  above  its  melting  point  becomes  a 
liquid  and  a  liquid  through  the  process  of  evaporation  or 
boiling  changes  into  a  gas.  It  is  also  possible,  at  cer- 
tain temperatures  and  pressures,  for  a  solid  to  pass 
directly  into  the  gaseous  state  and  vice  versa. 

From  the  present  point  of  view  the  cause  of  these 
changes  is  readily  seen.  We  have  said  that  heating  a 
body  means  increasing  the  violence  of  its  internal  vibra- 
tion. Now  it  is  easy  to  imagine  that  when  in  a  solid  sub- 
stance this  vibration  comes  to  exceed  a  certain  amount 
the  atoms  or  molecules  will  no  longer  be  able  to  adhere 
in  orderly  arrangement  but  will  be  forced  farther  apart 

[13] 


HEAT  PHENOMENA  [Chap.  IV 

by  the  motion  and,  although  not  completely  out  of  the 
influence  of  each  other's  attraction  so  that  they  become 
totally  dispersed,  still  are  so  far  apart  that  they  wander 
about  at  random,  like  the  frantic  members  of  a  mob. 
Under  these  circumstances  rigidity  no  longer  exists  and 
the  substance  is  liquid  (16). 

When  the  vibration  gets  still  more  violent,  i.e.,  when  the 
liquid  is  further  heated,  the  number  of  molecules  at  the 
surface  of  the  liquid  which  escape  into  the  surrounding 
region  becomes  very  large.  The  molecules  which  escape 
do  so  because  amid  the  random  vibration  they  happen  to 
have  a  speed  sufficient  to  carry  them  up  beyond  the  at- 
traction of  the  other  molecules  of  the  liquid.  These 
molecules  in  the  space  above  the  liquid  constitute  a 
gas  (17). 

In  the  case  of  a  solid  or  liquid  the  heat  motion  takes 
place  through  a  very  short  distance  and  is  then  reversed, 
and  therefore  we  may  speak  with  propriety  of  the  motion 
as  a  "vibration"  of  the  molecules  or  atoms.  In  the  case 
of  a  gas,  however,  the  motion  is  different,  each  molecule 
travelling  practically  in  a  straight  line  until  by  chance  it 
encounters  another  flying  molecule.  In  such  a  gas  as 
air  (which  is  mainly  a  mixture  of  oxygen  gas  and  nitrogen 
gas)  a  molecule  travels  on  the  average  through  a  distance 
several  hundred  times  its  own  diameter  before  it  strikes 
another  (14). 

The  existence  of  this  atomic  and  molecular  motion  is 
at  the  very  heart  of  the  modern  conception  of  matter.  A 
somewhat  extended  analogy  will,  therefore,  not  be  out 
of  place. 

Suppose  that  into  a  room  are  thrown  at  high  speed, 
through  an  open  door,  ten  thousand  tennis  balls,  and  that 
the  door  is  then  closed.  It  is  clear  that  the  balls  will  for 
a  time  bound  back  and  forth  among  themselves,  striking 

[14] 


Chap.  IV]  MODEL  OF  A  GAS 

the  walls  and  each  other.  If,  now,  we  make  the  ideal 
assumption  that  the  walls  of  the  room  and  the  balls  are 
perfectly  elastic,  that  is,  that  there  is  no  energy  lost  at 
any  of  the  impacts,  it  is  clear  that  the  bouncing  of  the 

/ 

A A- -& - 

/  \ 


-x     •'        \  A 

.     >'     \    l'\ 

\ 

!  A    * 


Fig.  9 

VAPOR  MOLECULES  AT  THE  SURFACE   OF  A  LIQUID 

As  explained  in  the  text,  the  vapor  which  rises  from  the  surface  of  any 
liquid  consists  in  molecules  which  are  shot  through  the  film  of  surface 
attraction.  Slow-moving  molecules  may  penetrate  the  liquid  surface  but 
be  returned  to  it  once  more  by  the  forces  of  attraction.  Fast-moving 
molecules,  however,  may  escape  permanently.  The  paths  described  by 
molecules  of  both  sorts  are  illustrated  above.  A  is  the  limit  at  which 
the  attraction  ceases  to  be  effective  for  a  molecule  moving  sufficiently 
fast  to  reach  this  line.  B  is  the  liquid  surface. 

balls  will  go  on  indefinitely.  Such  a  room  with  the  balls 
will  represent  in  a  general  way  a  small  vessel  filled  with 
a  gas.1 

Some  of  the  well-known  properties  of  gases  follow  in 
the  simplest  way  from  a  consideration  of  the  above  model 
of  a  gas.  It  is  obvious  that  the  walls  of  the  room  will  be 

1  The  analogy  here  given  may  appear  crude,  but  an  actual  model 
on  the  general  principle  outlined,  using  small  steel  balls,  has  been 
constructed  by  Professor  Northrup  of  Princeton.  When  in 
action,  this  model  exhibits  all  of  the  fundamental  properties  and 
laws  of  gases. 

[16] 


HEAT  PHENOMENA  [Chap.  IV 

bombarded  by  the  flying  tennis  balls.  The  walls  will, 
therefore,  feel  a  thrust,  that  is,  they  will  have  an  outward 
pressure  acting  upon  them.  This  corresponds  to  the  well- 
known  pressure  of  any  gas  confined  in  a  closed  vessel. 
Moreover,  this  pressure  due  to  bombardment  will  be 
greater  if  the  speed  of  the  flying  balls  is  greater,  and 
hence  in  the  case  of  a  gas  we  should  expect  the  pressure 
to  increase  with  the  temperature,  that  is,  with  the  aver- 
age energy  of  motion  of  a  molecule.  It  is  a  well-known 
fact  that  the  pressure  of  a  gas  does  increase  in  this  way 
(18),  (19). 

Again,  returning  to  our  model,  it  seems  fairly  reason- 
able to  suppose  (and  can,  in  fact,  be  proved  mathemati- 
cally) that  if  the  balls  hi  one  hah*  of  the  room  have  on 
the  average  a  higher  speed  than  the  balls  in  the  other 
half,  there  will  be  a  gradual  slowing  down  of  the  one  and 
a  speeding  up  of  the  other  until  all  of  the  balls  have  the 
same  average  speed.  This  corresponds  to  the  gradual 
equalization  of  temperature  which  goes  on  in  any  vessel 
containing  a  gas,  when  at  the  start  one  part  of  the  gas  is 
at  a  higher  temperature  than  another.  This  transmission 
of  energy  in  a  gas  is  what  is  called  heat  conduction  (20). 

The  Brotonian  Movement  and  the  Visibility  of  Heat 
Motion.  —  If  one  of  the  tennis  balls  above  mentioned  were 
much  larger  and  heavier  than  the  rest  it  would  be  found 
to  move  on  the  average  much  more  slowly.  In  the  case 
of  a  gas,  therefore,  heavy  molecules  move  on  the  average 
more  slowly  than  light  ones.  As  we  pass  to  heavier  and 
heavier  molecules  or  to  larger  and  larger  particles  of 
some  foreign  substance  immersed  in  the  gas,  the  average 
motion  of  the  particles  considered  becomes  less  and  less, 
until  it  disappears  into  the  imperceptible.  Now  the  ques- 
tion arises  whether  it  might  not  be  possible  to  detect  the 
motion  of  particles  so  large  that  they  could  be  seen  with 

[16] 


Chap.  IV]  MODEL  OF  A  LIQUID 

a  microscope.  If  this  were  possible  we  could  obtain  a 
direct  view  of  the  heat  motion  of  the  gas,  for  although  we 
should  not  be  able  to  see  the  motion  of  single  molecules 
we  could  see  the  closely  related  motion  of  a  very  much 
larger  particle. 

Now,  as  a  matter  of  fact,  this  continuous  random 
motion  of  all  very  small  solid  particles  floating  hi  a  gas 
or  a  liquid  is  a  well-known  phenomenon  and  is  called  the 
Brownian  Movement.  It  is  so  common  that  biologists 
have  to  learn  to  distinguish  between  the  life-motions  of 
bacteria  which  they  are  examining  under  the  micro- 
scope and  the  "Broionian  movements"  which  the  bacteria 
have  in  common  with  all  other  small  particles. 

This  Brownian  movement  is  an  extraordinary  veri- 
fication of  modern  ideas  of  heat  for  the  verification 
goes  farther  than  was  stated  above.  It  has  recently 
been  shown  that  the  motion  observed  under  the  micro- 
scope is  not  only  of  the  same  £*W  but  also  of  the  same 
magnitude  as  that  which  is  predicted  mathematically 
from  molecular  considerations  (21). 

A  Model  of  a  Liquid.  —  If  we  imagine  the  above  men- 
tioned tennis  balls  to  have  then-  average  motion  slowly 
diminished,  and  if  we  remember  that  in  order  to  repre- 
sent molecules  the  balls  must  have  a  slight  attraction 
for  each  other,  it  will  be  clear  that  finally  the  balls  will 
no  longer  fill  the  room  as  they  did  before  but  will  divide 
themselves  into  two  groups.  There  will  be  a  layer  of  balls 
on  the  floor  adhering  more  or  less  closely  together,  al- 
though still  vibrating  among  themselves,  and  above  this 
layer  there  will  be  flying  at  random  the  balls  which  we 
have  already  taken  to  represent  a  gas.  The  layer  on  the 
floor  might  be  a  foot  or  two  thick,  depending  on  the  num- 
ber of  balls  present,  and  they  would  give  us  the  impres- 
sion of  a  ceaselessly  squirming  mass. 

[17] 


HEAT  PHENOMENA  [Chap.  IV 

This  dense  layer  of  agitated  balls  on  the  floor  repre- 
sents the  surface  of  the  liquid  and  the  flying  balls  in 
the  remainder  of  the  room  represent  the  gas  or  vapor 
which  always  exists  above  any  liquid  confined  in  a  closed 
vessel. 

It  will  be  clear  hi  a  general  way  from  this  model  why 
"heat  expands"  in  the  case  of  a  liquid,  for  as  the  violence 
of  vibration  increases  (and  this  corresponds  to  a  rise  in 
temperature)  the  closely  adhering  balls  representing  the 
liquid  will,  on  the  average,  be  forced  farther  apart  and 
the  total  volume  of  balls  will  appear  to  fill  more  space. 

In  a  few  rare  cases  a  liquid  expands  on  being  cooled. 
In  these  cases  we  must  imagine  that  the  molecules 
are  not  simple  spheres  but  have  more  complicated 
shapes.  A  change  in  temperature  may  therefore  result 
in  a  different  fitting  together  and  unexpected  volume 
changes. 

A  Model  of  a  Solid.  —  The  model  above  discussed 
probably  represents  the  truth  in  a  general  way  as  re- 
gards a  liquid,  a  gas,  and  the  relation  between  the  two, 
but  we  cannot  be  so  sure  in  the  case  of  a  solid. 

The  tennis  balls,  if  they  are  to  represent  molecules, 
must  not  be  perfectly  spherical.  Let  us  suppose  them 
to  be  egg-shaped.  Let  us  suppose  too  that  the  internal 
motion  of  the  balls  on  the  floor  be  diminished  more  and 
more  and  that  after  a  while  there  is  a  tendency  to  form 
orderly  arrangements  which  persist.  They  will  still  be 
vibrating  somewhat,  but  if  one  of  the  balls  has  its  sharper 
end  turned  in  one  direction  at  present,  it  will  be  turned  hi 
the  same  direction  at  a  later  period.  It  is  as  if  a  net-work 
of  elastic  threads  fastened  the  balls  together.  They  are 
still  capable  of  vibrating  to  and  fro,  but  any  individual 
maintains  permanently  a  certain  position  in  the  total 
mass. 

[18] 


Chap.  IV]    COOLING  AND  HEATING  EFFECTS 

Such  a  model  probably  corresponds  to  a  substance  in 
the  solid  state,  although,  as  has  been  said,  this  is  not 
certain  at  present.  There  may  be  some  other  subtle 
difference  between  a  liquid  and  a  solid,  but  this  is  per- 
haps the  principal  one.  One  reason  why  we  are  not 
very  sure  that  permanent  orderly  arrangement  is  the 
only  difference  between  a  solid  and  a  liquid  is  because 
of  the  existence  of  orderly  arrangement  within  liquids. 
Liquid  crystals,  as  they  are  called,  are  known  to  exist  and 
certainly  are  of  extreme  interest  and  importance  (22). 

How  Friction  Causes  Heat.  —  It  will  be  clear  from  the 
above  considerations  why  the  rubbing  of  one  surface  on 
another  invariably  causes  heat.  The  molecules  of  both 
bodies  are  "stirred  up"  as  it  were,  so  that  the  violence 
of  vibration  is  increased  and  this  corresponds  to  a  rise  in 
temperature.  The  energy  required  to  increase  the  vi- 
bration of  the  molecules  comes  from  the  work  done  in 
the  rubbing. 

Why  "  Evaporation  Cools."  -  That  the  evaporation  of 
a  liquid  has  a  cooling  effect  is  well  known  to  everyone. 
Boys  detect  the  direction  of  the  wind  by  noticing  the 
coolness  of  one  side  of  a  wet  finger.  From  the  above 
consideration  the  reason  for  this  cooling  effect  is  not 
difficult  to  imagine.  From  the  surface  of  a  liquid,  mole- 
cules are  constantly  passing  away  to  become  part  of  the 
surrounding  vapor.  In  the  course  of  the  random  motion 
which  a  molecule  at  the  surface  of  a  liquid  undergoes  it 
may  at  certain  times,  as  has  been  said,  attain  sufficient 
speed  to  enable  it  to  break  away  from  the  attraction  of 
its  neighbors.  (See  Figure  9.)  Since  the  deserters  will 
always  be  molecules  which  at  the  moment  possess  greater 
speeds  than  the  average,  the  liquid  by  evaporation  is 
constantly  losing  some  of  its  fastest  moving  particles. 
Such  selective  action  will  result  in  a  gradual  decrease  in 

[19] 


HEAT  PHENOMENA  [Chap.  IV 

the  average  speed  of  those  that  remain.    This  corre- 
sponds to  a  cooling  of  the  liquid  (23). 

The  "Absolute  Zero."  —  It  has  been  said  that  from  the 
modern  point  of  view  the  violence  of  molecular  or  atomic 
vibration  corresponds  to  the  temperature  of  a  body. 
Now  if  we  imagine  a  body  to  be  cooled  indefinitely  it  is 
clear  that  sooner  or  later  we  should  reach  a  point  at 
which  all  vibration  would  have  ceased,  that  is,  when  the 
molecules  and  atoms  of  a  body  would  simply  be  packed 
together  in  an  absolutely  inert  mass.  Such  a  point  would 
correspond  to  a  temperature  below  which  it  would  be 
impossible  to  go,  because  it  is  obviously  impossible  to 
have  less  vibration  than  no  vibration. 

There  are  ways  (see  Section  18)  of  calculating  in  terms 
of  degrees  Fahrenheit  this  lowest  conceivable  tempera- 
ture although  it  cannot  be  completely  attained  in  practice. 
It  is  approximately  459°  below  zero.  This  temperature  is 
called  "the  absolute  zero"  and  corresponds  to  the  ab- 
sence of  all  heat. 

"Heat"  and  "cold"  are  consequently  not  symmetrical 
terms.  "Heat"  is  molecular  motion.  "Cold"  is  the 
absence  of  molecular  motion,  that  is,  the  absence  of  heat. 
It  is  therefore  wrong  to  speak  of  "adding  cold"  to  a 
body.  We  should  say,  "taking  heat  away."  The  value 
of  ice  in  a  refrigerator  consists  in  the  fact  that  it  absorbs 
large  quantities  of  heat  from  the  objects  put  near  it  and 
not  that  it  "gives  out  cold." 

The  Heat  Energy  in  Bodies.  —  From  the  above  con- 
siderations it  follows  that  all  bodies  at  room-temperature 
possess  enormous  quantities  of  heat  and  what  we  call  a 
"hot  body"  is  distinguished  from  a  "cold  body"  by  the 
fact  that  the  first  has  a  higher  temperature  than  the  hu- 
man body,  and  that  the  second  has  a  lower  temperature 
(24). 

[20] 


Chap.  IV]  HEAT  ENERGY 

It  may  be  worth  stating  that  the  amount  of  heat  in  a 
glass  of  water  at  ordinary  temperatures  corresponds  to  an 
amount  of  energy  which,  if  utilized  mechanically,  would 
be  sufficient  to  raise  this  water  to  a  distance  of  thirty 
miles  or  more  above  the  ground.  Practically  the  same 
would  be  true  of  a  piece  of  ice,  since  its  total  heat  is  only 
a  little  less  than  that  of  the  water. 

It  may  also  be  worth  mentioning  that  in  the  last  few 
years  temperatures  have  been  reached  in  the  laboratory 
which  are  only  two  or  three  degrees  above  the  absolute 
zero.  At  such  low  temperatures  some  of  the  properties 
of  matter,  as  we  shall  see  later  (Part  II,  Section  54), 
undergo  remarkable  modifications. 


CHAPTER   V 

THE  ELECTRON  AND   ITS  BEHAVIOR 

We  are  now  ready  to  consider  the  second  fundamental 
entity,  namely  the  electron.  To  quote  from  the  first 
chapter,  "  according  to  the  modern  theory,  all  bodies  are 
complex  structures  composed  of  small  particles  called 
atoms  and  still  smaller  particles  known  as  electrons." 

So  far  as  we  know,  all  electrons  are  exactly  alike.  In 
this  respect,  therefore,  they  differ  greatly  from  atoms, 
which,  it  will  be  remembered,  exist  in  about  a  hundred 
different  varieties. 

Size.  —  In  size  the  electron  is  very  much  smaller  than 
the  atom.  The  exact  size  is  not  known  but  it  has  a  di- 
ameter of  about  one  one-hundred-thousandth  that  of  an 
atom.  This  means  that  if  the  average  atom  be  repre- 
sented by  a  sphere  one  hundred  yards  in  diameter,  the 
electron,  on  the  same  scale,  would  be  about  the  size  of  a 
pin-head.  In  other  words,  a  large  office  building  is  not 

[21] 


THE  NATURE  OF  THE  ELECTRON     [Chap.  V 

large  enough  to  represent  an  atom  if  a  pin-head  is  to 
represent  an  electron. 

Weight. — The  electron  is  much  lighter  than  any  known 
atom  although  hi  proportion  to  its  size  it  is  much 
heavier.  Although  the  atom  is  enormously  larger  than 
the  electron,  the  lightest  atom,  namely  that  of  hydro- 
gen, is  only  about  two  thousand  times  as  heavy  as  an 
electron.1  A  short  calculation  shows,  therefore,  that 
the  " density"  of  the  electron  is  a  million  million  times 
that  of  the  atom. 

The  minuteness  of  the  electron  may  seem  almost  in- 
credible, but  careful  research  leads  almost  inevitably  to 
the  conclusions  stated,  and  the  scientist  must  report 
what  he  finds. 

Shape  and  Structure.  —  Practically  nothing  is  known 
as  to  the  shape  or  structure  of  the  electron.  There  are 
indications,  however,  that  it  is  spherical  in  shape  and 
symmetrical  hi  every  way  (25). 

The  Two  Electricities.  —  It  will  be  remembered  by  those 
whose  physics  is  not  too  distantly  lost  in  the  past  that 
there  are  two  "kinds"  of  electricity,  " positive"  and 
"negative."  If  a  body  is  charged  with  electricity  of 
one  kind  it  repels  all  bodies  having  a  similar  charge 
and  attracts  all  those  having  an  opposite  charge.  In 
the  familiar  terms:  "like  charges  repel  each  other; 
unlike  charges  attract  each  other."  The  attraction 
of  unlike  charges  is  the  common  phenomenon  noticed 
when  in  cold  weather  a  recently  used  comb  is  held 
near  bits  of  paper.  At  present  it  seems  not  improbable 
that  most  of  the  phenomena  in  nature  are  due,  in  the  last 
analysis,  to  electric  attractions  and  repulsions  (26). 

1  Strictly  speaking  it  is  the  "  mass"  and  not  the  weight  that  we 
refer  to,  but  the  term  "  weight"  is  in  common  use  and  of  proper 
implication. 

[22] 


Chap.  V]    NEGATIVE  AND  POSITIVE  ELECTRICITY 

Both  Kinds  of  Electricity  Abundant  in  all  Bodies.  —  All 
bodies  seem  to  possess  enormous  quantities  of  both 
positive  and  negative  electricity,  but  usually  it  is  in  exactly 
equal  amounts,  so  that  one  kind  neutralizes  completely  the 
effect  of  the  other  and  no  electricity  seems  to  be  present. 
Charging  a  body  with  electricity  is  then  to  be  considered 
as  merely  disturbing  this  balance  by  taking  away  or  add- 
ing to  the  body  a  small  amount  of  one  kind  of  electricity. 
We  shall  see  later  that  the  electricity  added  or  taken  away 
appears  hi  the  light  of  modern  theory  always  to  be  the 
negative. 

Electrons  Negatively  Charged.  —  Each  electron  has  a 
negative  charge  of  electricity  and  this  charge,  consider- 
ing the  size  of  the  particle,  is  very  great.  Electrons  are 
therefore,  attracted  towards  all  positive  charges  of  elec- 
tricity and  at  the  same  time  repel  each  other  strongly. 
Modern  research  has  made  it  probable  that  not  only  do 
electrons  always  possess  a  negative  charge  but  negative 
electricity  exists  only  in  the  form  of  electrons.  That  is, 
negative  electricity  and  electrons  are  inseparable  and  the 
only  way  to  give  a  body  a  negative  charge  is  to  put 
electrons  on  it  or  hi  it. 

Atoms  and  Electricity.  —  Since  all  bodies  are  made  up 
of  atoms,  charging  a  body  with  electricity  is  the  same  as 
charging  some  of  its  atoms  with  electricity.  Speaking 
now  of  " atoms"  instead  of  " bodies,"  it  follows  from 
the  above  that  no  atom  can  be  charged  with  negative 
electricity  without  putting  one  or  more  electrons  on  it. 
Each  ordinary  atom  contains  a  number  of  electrons  and 
enough  positive  electricity  to  exactly  balance  the  negative 
electricity  of  the  electrons.  At  present  it  appears  that 
the  positive  electricity  never  leaves  the  atom,  whereas  elec- 
trons allow  themselves  to  be  taken  away  from  or  added 
to  the  atom  with  relative  ease. 

[23] 


NATURE   OF  THE  ELECTRON         [Chap.  V 

Negative  Charge  "Too  Many"  Electrons;  Positive  Charge 
"  Too  Few."  -  Since  it  is  probable  that  only  negative 
electricity  in  the  form  of  electrons  is  movable,  an  atom 
can  be  charged  positively  only  by  taking  away  some  of 
the  electrons  which  it  normally  possesses.  This  allows 
the  positive  charge  of  the  atom  (which  it  has  perpetually) 
to  predominate  and  produces  the  same  effect  as  if  posi- 
tive electricity  had  been  added  to  it.  Thus  briefly,  an 
atom  contains  normally  a  certain  number  of  electrons 
and  also  positive  electricity  enough  to  neutralize  exactly 
their  negative  charges.  The  atom  is  then  "uncharged." 
If  an  electron  is  added  to  the  atom  from  the  outside  there 
will  be  more  negative  electricity  than  positive  and  the 
atom  will  have  a  " negative  charge"  (27). 

The  Electric  Current.  —  The  attraction  which  an  atom 
has  for  an  electron  varies  greatly  with  the  different 
species  of  atoms.  The  atoms  of  the  so-called  metals 
exert  only  a  relatively  weak  attraction  on  electrons, 
whereas  the  attraction  of  the  "non  metals"  appears  to 
be  greater.  In  a  metal,  therefore,  it  will  be  relatively 
easy  to  move  electrons  from  place  to  place. 

When  a  stream  of  electrons  is  caused  to  move  through 
the  body  of  such  a  substance  we  have  an  electric  current. 
From  the  modern  point  of  view,  therefore,  an  electric  cur- 
rent in  a  wire  is  a  stream  of  electrons  moving  through 
the  relatively  large  spaces  between  the  atoms  or  through 
the  atoms  themselves  (28). 

The  electrons  forming  the  electric  current  move  very 
slowly,  perhaps  only  several  inches  a  minute,  but  they 
move  in  enormous  numbers.  This  speed  must  not  be 
confused  with  the  so-called  "  speed  of  electricity  *' 
The  far  greater  " speed  of  electricity"  is  due  to  the  fact 
that  the  impulse  is  passed  on  very  rapidly  from  electron 
to  electron,  so  that  when  the  electrons  at  the  near  end  of 

[24] 


Chap.  V]  THE  ELECTRIC   CURRENT 

a  hundred  mile  wire  are  set  moving  those  at  the  distant 
end  are  caused  to  take  up  the  motion  a  very  small  frac- 
tion of  a  second  later.  Briefly,  the  actual  speed  of  the 
electrons  is  very  slow,  but  the  rate  of  transmission  of 
motion  from  electron  to  electron  is  very  great.  The 
action  is  closely  similar  to  what  follows  when  one  end  of 
a  long  rope  is  pulled.  The  impulse  which  results  in  the 
movement  of  the  other  end  travels  with  much  greater 
speed  than  the  rope  itself  commonly  attains. 

In  the  electrical  case,  the  impulse  to  move  travels  with 
the  speed  of  light,  i.e.,  one  hundred  and  eighty-six  thou- 
sand miles  a  second,  whereas  the  electrons  themselves 
(i.e.,  the  true  electricity)  move  only  a  small  fraction  of 
an  inch  a  minute. 

The  Action  of  a  Battery  or  Dynamo.  —  Since  it  appears 
probable  that  electricity  in  its  movable  state  always  con- 
sists of  one  or  more  electrons,  it  is  clear  that  no  machine 
or  device  of  any  kind  can  produce  electricity  (29).  What  it 
does  is  to  drive  electricity.  Hence  a  battery  might  be 
called  "an  electricity  pump"  or  perhaps  even  "an  elec- 
tron pump."  Because  of  the  chemical  action  taking  place 
within  the  battery  it  is  enabled  to  force  electrons  out 
through  its  negative  terminal,  and  these  electrons  flow 
through  the  wires  of  the  outside  circuit  and  re-enter  the 
battery  again  through  the  positive  terminal. 

We  pay  a  lighting  company,  therefore,  not  for  "elec- 
tricity" but  for  electrical  energy.  Nor  is  electricity 
"used  up"  when  the  current  passes  through  the  fila- 
ment of  an  incandescent  lamp.  Precisely  as  many  elec- 
trons leave  the  filament  as  enter  it,  but  the  stream  as  it 
passes  through  tends  to  set  the  atoms  hi  more  violent 
vibration,  and  so  heats  or  maintains  the  temperature  of 
the  filament. 

The  electric  transmission  of  power  is  thus  closely 
[25] 


NATURE  OF  THE  ELECTRON          [Chap.  V 

analogous  to  the  transmission  of  power  by  compressed 
air.  We  must  stipulate,  to  improve  the  analogy,  that  the 
compressed  air  when  "used"  at  the  far  end  of  the  pipe- 
line be  not  set  free,  but  returned  by  another  pipe  to  the 
air  compressor.  Under  these  conditions,  if  the  action 
goes  on  for  a  long  enough  time,  the  same  air  will  go 
several  times  around  the  circuit.  Ah*  is  not  consumed, 
nor  is  it  manufactured.  It  is  simply  compressed,  that  is, 
pumped  around  the  circuit.  The  " consumer"  who  pays 
for  compressed  air  under  these  circumstances  gets  value 
because  the  air  comes  to  him  at  a  high  pressure  and  he 
sends  it  back  at  a  low  pressure.  He  has  consumed 
energy  and  not  air. 

The  difference  in  ah*  pressure  in  the  pipes  corresponds 
to  the  " voltage"  of  an  electric  transmission  line  (30). 

"  Free  Electrons"  —  It  has  been  said  that  the  atoms  of 
different  elements  seem  to  have  different  attractions  for 
electrons.  Recent  experiments  have  made  it  probable 
that  the  so-called  "positive"  elements,  including  the 
metals,  have  a  relatively  weak  attraction  and  the  negative 
elements,  such  as  sulphur,  a  powerful  one.  Many  elec- 
trical phenomena  probably  owe  their  existence  to  this 
fact. 

It  is  doubtless  owing  to  this  difference,  for  example, 
that  metals  conduct  electricity  so  readily,  whereas  sub- 
stances like  sulphur  do  not.  We  must  suppose  that  the 
atoms  of  a  metal  have  such  a  weak  attraction  for  electrons 
that  a  vast  number  of  the  latter  are  in  a  practically  free 
state  throughout  the  body  of  the  metal  and  are  thus 
capable  of  being  moved  readily  by  any  outside  electric 
forces.  This  ease  of  movement  makes  the  substance  a 
good  conductor.  Atoms  of  such  elements  as  sulphur,  on 
the  other  hand,  possess  such  great  attraction  for  elec- 
trons that  most  of  them  are  held  tight  in  the  atoms  and 

[26] 


Chap.  VI]  FREE  ELECTRONS 

cannot  be  moved  easily  from  one  place  to  another  within 
the  substance.  This  makes  the  material  a  "poor  con- 
ductor," or,  as  we  say,  a  "good  insulator." 

There  is  good  reason  for  believing  that  the  electrons 
within  a  conductor  act  as  regards  heat  motion  as  if  they 
were  small  atoms,  that  is,  they  take  part  with  the  atoms  or 
molecules  hi  the  random  vibration  which  appears  to  con- 
stitute the  heat  of  a  body. 

The  "Evaporation"  of  Electrons.  —  If  electrons  exist  in 
large  numbers  within  the  substance  of  a  metal  it  might 
be  expected  that  if  a  metal  were  heated  hot  enough  some 
of  these  would  be  given  off  into  the  surrounding  space, 
after  the  manner  in  which  a  liquid  loses  molecules  by 
evaporation.  In  fact  this  is  found  by  experiment  to  be  the 
case.  The  emission  of  electrons  appears  to  be  in  every 
way  analogous  to  the  evaporation  of  a  liquid  (31). 


CHAPTER    VI 
ELECTRONS,  CHEMICAL  ACTION,  AND  LIGHT 

Electrons  and  Chemical  Action.  —  It  seems  probable 
that  the  forces  involved  in  chemical  affinity  are  electrical 
in  character,  that  is,  the  atoms  which  form  the  groups 
known  as  molecules  are  held  together  by  electric  attrac- 
tion. Thus  a  molecule  of  hydrochloric  acid  is  composed 
of  one  atom  of  hydrogen  and  one  atom  of  chlorine,  and 
the  two  cling  together,  probably  because  the  chlorine  atom 
has  a  negative  charge,  while  the  hydrogen  atom  has  a 
positive  one,  and  "unlike  charges  attract  each  other." 

When  hydrogen  gas  and  chlorine  gas  are  put  together 
in  a  vessel,  heat  and  even  light  will  cause  them  to  combine 
suddenly  and  to  form  hydrochloric  acid.  That  is,  each 
atom  of  one  kind  becomes  attached  to  one  of  the  other 

[27] 


ELECTRONS  AND  LIGHT 


[Chap.  VI 


Fig.  10 


kind,  forming    a   molecule   of  the  new   "compound," 
hydrochloric  acid. 

We  are  on  rather  treacherous  ground  at  this  point,  but 
we  shall  probably  be  not  far  wrong  if  we  picture  the 

mechanism  of  the  process  of 
union  somewhat  as  follows. 
The  light  or  heat  detaches 
from  some  of  the  atoms  a  few 
electrons  and  these  bound 
about  at  random  between  the 
molecules  of  the  two  sepa- 
rate gases.  A  very  important 
fact  then  makes  itself  felt. 
As  was  said  in  the  last  chap- 
ter, different  kinds  of  atoms 
have  very  different  attrac- 
tions for  electrons,  and,  hi 
the  present  case,  the  attrac- 
tion of  the  chlorine  atoms  is 
vastly  greater  than  that  of 
the  hydrogen.  Thus  it  will  happen  before  long,  since 
a  few  new  electrons  are  being  detached  constantly,  that 
every  chlorine  atom  has  one  electron  too  many  while  every 
hydrogen  atom  has  one  electron  too  few.  This  means,  of 
course,  that  each  of  the  former  attains  a  negative  charge, 
and  each  of  the  latter  a  positive  one.  The  remainder  of 
the  process  consists  in  the  attraction,  and  permanent 
combination  in  twos,  of  these  atoms  of  unlike  charge  to 
form  the  groups  which  we  call  hydrochloric  acid  mole- 
cules (32),  (33),  (34). 

This  theory  of  chemical  action  is  not  certain  as  yet 
but  is  worth  mentioning. 

It  is  to  be  noticed  that  some  kind  of  disturbance,  in 
the  above  case  heat  or  light,  is  necessary  to  keep  up  the 

[28] 


THE   CONSTITUTION   OF  A 
SIMPLE   MOLECULE 

The  molecule  which  is  symbolically 
represented  above  is  one  of  hydro- 
chloric (muriatic)  acid.  As  shown, 
it  is  made  up  of  one  atom  of  hydro- 
gen, //,  combined  with  one  atom 
of  chlorine,  CL  The  former  bears 
a  positive  electrical  charge,  and  the 
latter  an  equal  negative  charge.  It 
is  the  attraction  between  these  op- 
posite charges  which  is  supposed  to 
hold  the  molecule  together.  When 
the  charged  atoms  are  separated, 
as  in  "electrolytic  dissociation"  (see 
text),  they  form  hydrogen  and  chlo- 
rine "ions." 


Chap.  VI]    ELECTRONS  AND  CHEMICAL  ACTION 

supply  of  "free  electrons."  We  see,  therefore,  that 
were  there  no  heat  or  light,  or  were  the  intensity  of 
these  below  a  certain  limit,  depending  on  the  nature  of 
the  substances,  we  could  get  no  chemical  action  (35). 
This  inertness  would  probably  be  a  property  of  all  sub- 
stances in  the  dark  at  the  so-called  absolute  zero  of 
temperature  (36). 

Atoms,  Electrons  and  Light.  —  There  is  good  reason  for 
believing  that  light-waves  are  electrical  hi  character. 
There  seems  to  be  no  fundamental  difference  between 
light-waves  and  the  electric  waves  used  in  wireless  teleg- 
raphy, except  that  the  latter  are  very  much  "longer" 
and  the  vibration  is  very  much  slower  than  in  the  former 
case.  Light-waves  and  "wireless"  waves  are  thus  re- 
lated in  the  same  way  that  a  high-pitched  sound  is 
related  to  one  of  low  pitch. 

"Wireless"  waves  (i.e.,  "Hertz"  waves)  are  always 
produced  by  causing  a  charge  of  electricity  to  oscillate  to 
and  fro.  According  to  the  prevailing  view,  waves  are 
thus  set  up  hi  the  "ether  of  space  "  in  a  manner  somewhat 
similar  to  the  way  sound-waves  are  set  up  by  a  vibrating 
bell  (37). 

Since  an  oscillating  charge  is  thus  the  cause  of  these 
waves  it  seems  reasonable  to  ask  what  electric  charge  is 
responsible  for  the  closely  similar  but  vastly  more  rapid 
waves  of  light.  This  question  has  been  asked,  and  an- 
swered by  studying  the  effect  of  a  powerful  magnet  on 
various  sources  of  the  vibrating  charges  within  the  glow- 
ing body  which  must  be  held  responsible  for  the  light- 
waves emitted.  Results  obtained  by  a  distinguished 
Dutch  scientist  lead  to  the  conclusion  that  the  charge  is 
that  of  the  electron.  Here  again,  therefore,  we  are  thrown 
back  on  the  same  fundamental  entity  (38). 

The  "radiant  heat"  from  the  sun  is  also  of  this  electric- 
[29] 


ELECTRONS  AND   LIGHT  [Chap.  VI 

wave  type  of  vibration  so  that  the  sun  must  be  considered 
a  light  and  heat  radiator  because  of  the  vast  number  of 
vibrating  electrons  which  it  contains. 

To  sum  up:  according  to  the  modern  wave  theory,  whenever 
an  electric  charge,  whether  this  charge  be  that  of  one  electron 
or  many,  vibrates  back  and  forth,  it  radiates  electrical  waves 
which  go  out  in  all  directions  in  space  (39).  If  the  oscil- 
lation is  very  slow  they  are  called  Hertzian  waves,  or 
"wireless"  waves.  If  the  oscillation  is  more  rapid  they 
are  in  general  termed  "  heat-waves,"  and  if  the  vibration 
is  still  more  rapid  the  waves  are  capable  of  affecting  the 
retina  of  the  eye,  and  are  called  "  light-waves "  (40). 

All  of  these  waves  can  be  absorbed,  refracted  and 
reflected.  They  all  transmit  energy  and  hence  are 
capable  of  heating  any  body  which  absorbs  them. 

The  Absorption  of  Electric  Waves.  —  The  mechanism 
of  the  absorption  of  electric  undulations  is  to  be  thought  of 
as  follows.  As  a  sound-wave  which  is  emitted  from  a 
vibrating  body  tends  to  set  any  body  which  it  strikes  into 
a  similar  vibration,  so  an  electric  wave  which  is  emitted 
by  an  oscillating  electric  charge  tends  to  set  vibrating  the 
electric  charges  within  the  body  which  it  strikes.  If 
these  charges  are  so  conditioned  that  they  are  capable  of 
responding  easily  to  the  particular  rapidity  of  vibration 
which  thus  strikes  them,  they  will  be  set  into  violent 
vibration  at  the  expense  of  the  energy  of  the  entering 
wave.  This  vibration  ultimately  becomes  the  random 
motion  of  heat. 

Since  absorption  is  apparently  due  to  motion  of  the 
electrons  which  a  body  contains,  it  follows  that  opaque- 
ness in  bodies  must  be  ascribed  to  this  electron  mobility. 
It  is  probable  that  if  the  electric  charges  within  a  body 
could  be  held  fixed,  the  latter  would  be  transparent  to  any 
electric  wave. 

[30] 


Chap.  VI]    PROPERTIES   OF  ELECTRIC  WAVES 

The  Reflection  of  Electric  Wanes.  —  When  the  electrons 
within  a  body  are  set  into  vibration  by  an  impinging 
light-wave  they  will,  of  course,  act  as  radiators  of  new 
light-waves,  and  if  the  body  has  a  flat  surface  those 
emitted  from  the  electrons  near  the  surface  will  join  to 
form  a  definite  single  wave  which  travels  back  in  an 
opposite  direction  to  that  of  the  entering  one.  This 
constitutes  the  reflected  wave,  which  exists  in  general 
whenever  light  strikes  a  flat  surface.  The  reflected 
wave  may  be  almost  as  strong  as  that  entering  or  it 
may  be  very  weak,  but  except  hi  ideal  cases  it  always 
exists. 

Whether  the  surface  is  flat  or  not  there  will  always  be 
reflection,  but  if  the  surface  has  a  certain  degree  of  flat- 
ness the  reflection  will  not  be  "diffuse"  like  that  from  a 
white-washed  wall,  but  will  be  regular,  like  that  from  a 
mirror  (41). 

The  Speed  of  Electric  Waves  in  Different  Bodies.  —  It 
is  a  generally  accepted  fact  that  all  electric  waves  whether 
they  correspond  to  light,  heat  or  Hertzian  waves,  travel 
with  the  same  speed  in  empty  space,  but  with  different 
speeds  in  material  bodies.  The  explanation  of  this  is 
that  all  material  bodies  contain  electrical  charges  and  that 
the  wave  which  passes  through  the  body  is  a  complex 
resultant  of  the  original  entering  wave  and  the  secondary 
waves  which  are  set  up  when  the  charges  oscillate  in 
response  to  the  entering  wave.  This  complex  resulting 
wave,  although  it  has  the  same  vibration  frequency  as 
the  original  one,  is  altogether  differently  conditioned  and 
it  can  be  shown  mathematically  that  it  will  not  travel  with 
the  same  speed  (42). 


[31] 


CHAPTER   VH 

ELECTRONS  AND  MAGNETISM 

The  Connection  of  Electricity  with  Magnetism.  —  It  is 
a  well-known  fact  that  magnetism  always  exists  in  the 
region  surrounding  a  wire  carrying  an  electric  current. 
Since  the  trend  of  modern  theory  is  towards  the  conclu- 
sion that  an  electric  current  always  consists  in  the  bodily 
movement  of  electrons  or  atoms,  we  must  suppose  mag- 
netism to  accompany  invariably  the  motion  of  electric 
charges,  and  indeed  this  has  been  found  to  be  true  (43). 

Any  magnetic  effect  can  best  be  magnified  by  winding 
the  wire  which  is  to  carry  the  electric  current  around  a 
piece  of  iron  in  the  same  general  way  that  thread  is 
wound  upon  a  spool.  The  wire  must,  of  course,  be  in- 
sulated so  that  the  various  turns  do  not  touch  each  other. 
Such  an  iron  spool  may  become  a  strong  magnet  when 
a  current  of  electricity  is  sent  through  the  wire  surround- 
ing it.  This  is  the  principle  at  the  basis  of  the  action  of 
the  ordinary  electric  motor.  The  electric  car  is  kept 
moving  because  the  current  which  enters  through  the 
trolley  wire,  and  leaves  through  the  track,  passes  through 
a  wire  wound  on  a  similar  spool,  and  the  spool  (then  a 
magnet)  through  its  attraction  sets  rotating  other  pieces 
of  iron  which  are  attached  to  the  wheels  of  the  car. 

Deflection  of  Electrons  Caused  by  Magnetism.  —  It  can  be 
shown  that  if  an  electric  charge  is  caused  to  move  rapidly 
past  the  end  of  a  magnet,  the  charge  tends  to  be  de- 
flected sidewise.  This  lateral  deflection  of  electrons  in 
a  so-called  " magnetic  field"  corresponds  to  one  of  the 
fundamental  relations  between  electricity  and  magnetism, 

[32] 


Chap.  VII]    ELECTRONS  AND   MAGNETISM 

and  by  its  means  we  have  an  important  method  of  "  gen- 
erating" an  electric  current,  i.e.,  of  moving  electrons. 
Suppose  for  instance  that  a  piece  of  wire  is  moved  rapidly 
through  a  magnetic  field,  that  is,  near  a  magnet.  The 
piece  of  wire  in  common  with  all  conductors  contains 
countless  easily  moved  electrons,  and  when  the  wire  is 
moved  through  the  field  the  deflecting  force  mentioned 
above  causes  them  to  be  pushed  towards  one  end  of  the 
wire,  If  the  latter  is  held  in  the  proper  position  (44). 

The  Action  of  a  Dynamo.  —  The  ordinary  dynamo  or 
electric  generator,  as  it  is  often  called,  is  a  machine  for 
moving  wires  rapidly  through  a  magnetic  field,  and  for 
collecting  the  electric  current  which  is  set  up  by  the  de- 
flecting forces  above  mentioned.  Such  a  machine  has, 
of  course,  to  be  run  by  some  outside  source  of  power  such 
as  a  steam-engine,  or  a  water-wheel. 

Permanent  Magnetism.  —  The  familiar  type  of  magnet 
which  is  not  maintained  by  means  of  an  electric  current, 
the  small  red-painted  horse-shoe  magnet  of  the  toy-shops, 
for  example,  is  called  technically  a  "  permanent  magnet." 
It  is  probable  that  the  ultimate  cause  of  magnetism  in 
this  case  is  quite  similar  to  that  in  the  case  of  the  spool 
before  mentioned.  In  the  latter,  electrons  circulate  in  a 
helix  around  the  outside  of  a  large  piece  of  metal,  and  in 
the  former,  although  there  is  no  circulation  of  electrons 
around  any  visible  path,  helical  or  circular,  still  such 
paths  probably  exist  within  the  molecules  of  the  iron  which 
forms  the  permanent  magnet.  According  to  the  present 
belief,  the  electrons  within  a  piece  of  magnetized  iron  do 
not  move  in  a  wholly  random  fashion,  but  have  what  we 
might  call  a  "  helical  prejudice,"  and  perhaps  even  re- 
volve in  circles  within  the  molecules  themselves. 

It  is  now  generally  believed  that  the  cause  of  magnetism 
is  always  the  motion  of  electrical  charges  (45). 

[33] 


RADIO-ACTIVITY  [Chap.  VHI 

The  Effect  of  Magnetism  on  Light.  —  Since  the  source 
of  the  light  which  an  incandescent  body  emits  lies  in  the 
vibratory  motions  of  electrons  within  the  body,  and  since 
magnetism  has  a  tendency  to  deflect  moving  electrons, 
we  should  not  be  surprised  to  find  that  certain  sources  of 
light  when  put  near  a  powerful  magnet  have  their  emitted 
light  modified  in  a  complicated  way.  This  has  been  found 
to  be  the  case,  and  the  results  tend  to  verify  the  views 
herein  set  forth  (38). 


CHAPTER   VHI 
RADIO-ACTIVITY 

There  is  a  remarkable  group  of  substances  which  emit 
rays  continuously,  without  obtaining  energy  from  their 
surroundings.  Radium  is,  perhaps,  the  most  striking 
member  of  this  group  and  it  is  now  known  with  consider- 
able surety  that  in  its  case  the  rays  would  only  fall  off  to 
about  half  then*  present  intensity  in  two  thousand  years. 
This  extraordinary  action  goes  under  the  general  name  of 
" radio-activity"  and  the  substances  which  emit  the  rays 
are  known  as  "radio-active  substances"  (46). 

The  Three  Rays.  —  The  rays  emitted  are  not  all  of 
the  same  kind,  but  are  composed  of  three  distinct  types, 
each  with  definite  characteristics  of  its  own.  It  was 
thought  convenient  in  the  beginning  to  designate  these 
rays  by  the  first  three  of  the  Greek  letters,  and  they  are 
therefore  known  as  the  alpha  rays,  the  beta  rays,  and  the 
gamma  rays. 

The  Beta  Rays.  —  There  is  now  no  reasonable  doubt 
that  the  beta  rays  consist  of  a  stream  of  electrons  com- 
ing out  of  the  substance  like  bullets  from  a  machine  gun. 
The  most  surprising  thing  about  these  rays  is  the  enor- 

[34] 


Chap.  VHi:  RADIUM  RAYS 

mous  velocity  of  the  electrons.  Their  speed  is  almost 
equal  to  that  of  light,  or  about  one  hundred  and  eighty- 
six  thousand  miles  a  second.  This  inconceivable  velocity 
is  measured  indirectly,  but  by  methods  which  make  it 
almost  certain  that  the  result  is  correct.  This  speed 
enables  part  of  the  rays  to  penetrate  a  plate  of  metal  as 
thick  as  an  ordinary  book  cover  (47). 

The  Alpha  Rays.  —  Within  the  last  few  years  it  has 
become  practically  certain  that  these  rays  consist  of  a 
stream  of  atoms  of  the  element  helium.  This  surprising 
fact  could  certainly  not  have  been  predicted  when  first 
the  rays  were  discovered.  The  helium  atoms  have  each 
a  double  positive  charge ;  each  atom,  that  is,  has  lost  two 
electrons.  The  atoms  of  the  stream  have  a  velocity  of 
about  one-tenth  that  of  light,  or  about  eighteen  thou- 
sand miles  per  second.  This  is  less  than  the  speed  of 
the  beta  rays,  but  it  must  be  remembered  that  the  atom 
of  helium,  whose  " atomic  weight"  is  four,  is  about  eight 
thousand  times  as  heavy  as  an  electron,  and  thus,  al- 
though the  helium  atoms  of  the  alpha  rays  are  moving 
more  slowly,  their  energy  is  much  greater  than  that  of 
the  beta  ray  particles,  the  electrons  (48). 

This  energy  is  in  fact  so  great  that  a  single  "alpha 
particle,"  that  is,  a  single  atom  of  helium,  when  it  strikes 
certain  substances,  will  cause  them  to  phosphoresce 
(or  more  properly  fluoresce)  for  an  instant  over  a  region 
large  enough  to  be  seen  with  a  lens.  This  makes  visible 
the  effect  due  to  a  single  atom  and  gives  us  the  first  case 
in  the  history  of  science  where  any  effect  caused  by  one  atom 
has  been  observed. 

The  Gamma  Rays.  —  There  has  been  some  dispute  as 
to  the  ultimate  nature  of  the  gamma  rays,  but  it  is  now 
practically  certain  that  they  do  not  consist  of  particles  in 
the  ordinary  sense,  but  of  impulses  similar  to  those  con- 

[36] 


RADIO-ACTIVITY 


[Chap.  VHI 


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Fig.  lla 

THE  RADIO-ACTIVE  ELEMENTS  AND  THEIR 
RELATIONSHIPS  AND    RAYS 

These  diagrams  represent  symbolically  the  four  series  of  radio-active  substances. 
In  each  vertical  line  the  elements  above  break  down  to  produce  those  immediately 
below,  as  indicated  by  the  arrows.  During  this  decomposition  they  emit  the  types 
of  radiation  designated  by  the  Greek  characters  attached  to  the  horizontal  arrows. 
Some  of  the  substances,  such  as  Meso-Thorium  1  and  Actinium,  are  rayless.  The 
periods  of  time  indicated  beside  each  element  stand  for  the  intervals  required  in 
order  that  half  of  a  given  quantity  of  the  element  in  question  should  have  decom- 
posed. Thus,  if  we  set  aside  a  gram  of  Radium  to-day  it  will  amount  to  only  half  a 
gram  when  two  thousand  years  have  elapsed.  The  other  half  will  have  gone 
through  the  change  symbolized  in  the  diagram.  The  figures  inside  the  circles  are 
the  respective  atomic  weights ;  those  outside,  the  atomic  numbers. 

[36] 


Chap.  Vffl]  RADIO-ACTIVE  SERIES 


25^*25  **<Su* 


26.S  Jays 


ay 


0.  00  J  second 


L/&AN/UM  2. 


/OA//L/M          nx,  ^^LKV 
Z00,000years   "  ^0^  Ot 


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Jd. 5  minutes 


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Fig.  lib 
[37] 


RADIO-ACTIVITY  [Chap.  VIE 

stituting  the  X  Rays  and  not  very  dissimilar  to  the  waves 
or  impulses  making  up  light. 

The  connection  between  the  beta  rays  and  the  gamma 
rays  is  probably  similar  to  that  between  the  bullet  and  the 
sound  in  the  case  of  a  gun.  Both  the  bullet  and  the  sound 
of  the  explosion  travel  away  from  the  mouth  of  the  gun, 
but  the  bullet  is  a  moving  object  while  the  sound  is  a  dis- 
turbance, or  a  wave  impulse  in  the  air.  In  perhaps  the 
same  general  way  both  the  beta  rays  and  the  gamma 
rays  come  out  of  the  radio-active  substance,  but  the 
beta  particle  is  a  moving  electron  while  the  gamma 
rays  are  probably  electric  wave  impulses.  Both  rays, 
however,  like  the  bullet  and  sound,  leave  the  source 
together  (49). 

Cause  of  Radio- Activity.  —  It  is  now  generally  believed 
that  the  cause  of  radio-activity  is  the  actual  breaking  up 
of  the  atoms  of  the  radiating  substance.  The  radium 
atom  " explodes,"  like  a  bomb,  into  two  "pieces"  and 
the  energy  of  the  explosion  gives  each  "piece"  a  high 
velocity.  The  two  pieces  are  a  helium  atom  and  an  atom 
of  an  element  previously  unknown.  The  new  element 
is  called  "niton"  or  "radium  emanation"  and  appears 
to  be  a  heavy  gas  belonging  to  the  same  group  in  the 
periodic  system  of  elements  as  do  helium  and  argon. 

We  have  then,  in  radio-activity,  a  case  of  natural 
alchemy,  but  since  it  has  been  found  impossible,  so  far, 
to  hasten  or  retard  the  process  artificially,  the  knowledge 
of  radio-activity  would  not  have  helped  the  alchemists  in 
their  quest. 

Successive  Disruptions  of  Radio-Active  Atoms.  —  The 
radium  atom  appears,  in  fact,  to  be  a  member  of  a  chain 
of  "explosive  atoms"  for  the  atom  of  "niton"  itself  is 
radio-active.  If  given  time,  it  will  itself  "explode"  and 
give  off  another  helium  atom.  The  chain  has  been  fol- 

C38] 


Chap.  VHI]    ENERGY  OF  RADIO-ACTIVITY 

lowed  out  through  nearly  a  dozen  elements,  as  shown  in 
Figure  11. 

Radio-Activity  Not  a  Chemical  Change.  —  It  must  be 
borne  in  mind  that  the  change  accompanying  the  process 
of  radio-activity  is  not  a  chemical  change.  As  has  already 
been  stated,  a  chemical  change  involves  the  regrouping  of 
the  atoms  to  form  new  molecules.  This  does  not  affect 
the  integrity  of  the  individual  atoms  but  only  their  rela- 
tions to  then*  neighbors.  In  radio-activity,  on  the  other 
hand,  we  have  changes  which  involve  the  actual  disrup- 
tion of  the  atom. 

Intra-Atomic  Energy. —  When  coal  is  burned,  the  energy 
is  derived  from  the  attraction  existing  between  the  atoms 
of  carbon  (coal)  and  the  oxygen  atoms  of  the  air,  and  it 
should  be  evident  that  hi  general  whenever  there  are 
strong  attractions  or  repulsions  between  different  atoms, 
there  is  a  possibility  of  obtaining  energy  by  allowing  these 
forces  to  act.  The  energy  developed  in  radio-activity 
has  also  its  origin  in  forces  acting  within  the  substance, 
but  they  are  not  the  forces  acting  between  the  atoms,  as  is 
the  case  with  all  chemical  energy,  but  those  acting  within 
the  atom,  the  so-called  "  intra-atomic  forces." 

We  may  therefore  say  with  truth  that  all  the  energy 
obtained  in  the  world  from  fuel  is  of  the  extra-atomic 
variety,  while  the  energy  of  radio-activity  is  something 
quite  new.  It  comes  out  of  the  atom  and  is  therefore 
intra-atomic  energy. 

The  Quantity  of  Intra-Atomic  Energy.  —  One  of  the 
most  remarkable  facts  about  these  intra-atomic  forces  is 
then*  great  size  compared  with  the  ex/ra-atomic  forces 
of  chemical  action.  The  slowness  of  the  radio-active 
changes  tends  to  conceal  this  but  it  is  nevertheless 
true. 

The  energy  given  off  by  radium  ultimately  becomes 

[39: 


RADIO-ACTIVITY  [Chap.  VIII 

heat  and  this  heat  has  been  directly  measured  a  number 
of  times.  Since  the  radium  would  be  hah*  gone  in  two 
thousand  years,  a  simple  calculation  shows  that  the 
total  energy  given  up  by  radium  and  its  products  during 
transformation  is  about  a  quarter  of  a  million  times  the 
energy  to  be  obtained  by  burning  an  equal  weight  of  coal.  To 
make  this  statement  a  little  more  vivid,  the  fact  might  be 
noted  as  an  illustration  that  a  large  ocean  steamship 
could  make  one  ocean  passage  on  the  energy  from  a  few 
pounds  of  radium  and  its  products  (50). 

The  Radio-Active  Elements.  —  There  appears  to  be  noth- 
ing unusual  about  the  radio-active  elements  except  their 
radio-activity.  For  example,  referring  to  Figure  11, 
radium  itself  belongs  to  the  same  chemical  family  as 
barium  and  behaves  chemically  like  barium.  The  next 
element,  the  atoms  of  which  are  formed  from  the  dis- 
ruption of  the  radium  atom  is  the  gas  called  " niton" 
(radium  emanation)  already  mentioned.  The  following 
one,  "  radium  A,"  so  called,  is  a  solid  substance. 

The  atomic  weight  of  radium  is  about  226,  that  of 
niton  about  222  and  that  of  "radium  A"  about  218. 
These  numbers  correspond  with  the  fact  that  the  alpha 
particle  (the  helium  atom)  which  comes  off  at  each  ex- 
plosion has  an  atomic  weight  of  4,  so  that  each  of  the 
above  elements  has  a  weight  of  4  less  than  the  element 
from  which  it  sprung. 

Are  All  of  the  Elements  Radio- Active?  —  There  is  some 
reason  to  believe,  although  as  yet  no  proof,  that  all  ele- 
ments are  decomposing  in  the  same  way,  or  in  other  words, 
that  radio-activity  is  a  universal  property  of  matter.  This 
means,  of  course,  that  in  the  case  of  the  well-known 
" permanent"  elements  the  process  is  so  slow  as  not  to 
be  noticeable.  Already  potassium  has  shown  signs  of 
being  radio-active  and  as  methods  of  measurement  be- 

[40] 


Chap.  VIII]       EVOLUTION  OF  ELEMENTS 

come  gradually  more  delicate  it  is  not  improbable  that 
other  elements  will  do  the  same  (51). 

The  Evolution  of  the  Elements.  —  The  probability  that 
all  elements  are  to  some  degree  radio-active  has  led  to 
the  general  conception  of  what  is  called  the  "evolution 
of  the  elements."  According  to  this  view  there  is  a  slow 
forming  of  the  lighter  elements  through  the  disintegra- 
tion of  the  heavier  ones.  The  well-known  elements  are 
to  be  considered  as  merely  those  whose  average  "life" 
is  so  great  as  to  make  them,  from  a  human  point  of  view, 
permanent.  If  this  conception  be  a  true  one,  if  the  atoms 
of  any  element  are  descendants,  as  it  were,  of  the  atoms 
of  heavier  elements,  then  it  is  not  difficult  to  understand 
in  a  general  way  why  the  well-known  relations  of  the 
periodic  system  exist  between  the  elements. 

One  objection  which  has  been  raised  to  this  view  is 
that,  did  such  evolution  exist,  it  must  have  completed 
itself  years  ago  and  hence  only  the  lighter  elements 
should  now  remain.  There  are,  however,  some  faint 
indications  that  under  conditions  differing  greatly  from 
those  on  the  earth,  such  namely  as  exist  in  the  hotter 
stars,  the  reverse  process  may  be  going  on.  This  would 
involve  the  building  up  of  the  heavier  elements  out  of 
the  lighter  ones.  At  present,  however,  such  a  sugges- 
tion is  mere  speculation  (52). 

CHAPTER   IX 
THE  STRUCTURE  OF  THE  ATOM 

The  study  of  radio-activity  opened  the  door  to  the 
knowledge  of  intra-atomic  phenomena  and  recently  it 
has  given  us  some  definite  ideas  as  to  the  details  of 
atomic  structure. 

[41] 


ATOMIC  STRUCTURE  [Chap.  IX 

General  Principles.  —  One  or  two  general  statements 
must  be  made  at  the  outset.  In  the  first  place,  electricity 
certainly  plays  a  highly  important  role  in  the  formation 
of  the  atom  and  the  intra-atomic  forces  must  be  largely 
electrical.  Whether  all  intra-atomic  force  is  of  this  nature 
is  a  question  which  cannot  be  answered  at  present. 

Secondly,  we  can  say  without  much  chance  of  ultimate 
contradiction  that  the  atom  is  very  much  more  "porous," 
or  "open  work,"  in  structure  than  the  appearance  of  the 
vast  aggregates  of  atoms  which  we  know  as  "bodies," 
would  lead  us  to  believe.  It  has  been  stated  that  some  of 
the  beta  particles  (electrons)  from  radium,  travelling  with 
nearly  the  velocity  of  light,  go  straight  through  a  piece  of 
metal  as  thick  as  the  average  book  cover,  and  it  needs 
but  a  very  short  calculation  to  show  that  this  means  that 
the  electron  penetrates  more  than  a  million  atoms  one 
after  the  other.  It  must  be  remembered  that  in  a  solid 
substance  the  atoms  are  quite  near  together  so  that  such 
a  flying  electron  has  no  chance  of  travelling  entirely  in 
the  space  between  the  atoms  during  its  whole  passage 
through  the  metal. 

If  the  atom  is  made  up  almost  entirely  of  electrons,  it  is 
easy  to  show  that,  on  the  average,  the  electrons  must  be 
as  far  apart  compared  with  their  size  as  are  the  planets 
in  the  solar  system.  This  at  first  sight  seems  incredible, 
but  it  is  not  more  so  than  many  of  the  accepted  facts  of 
astronomy. 

The  Evidence  for  Orderly  Structure  in  the  Atom;  Spectral 
Lines.  —  Although  much  of  the  light  and  heat  given  out 
by  bodies  seems  to  be  due  to  the  random  vibration  of  the 
electrons  within  them,  this  is  by  no  means  invariably  the 
case.  Almost  all  gases  when  caused  to  give  out  light,  emit 
it  in  the  form  of  "pure  notes,"  that  is  in  the  form  of  waves 
of  definite  frequency.  The  light  which  we  get  from  a 

[42] 


Chap.  IX]  SPECTRAL  LINES 

white-hot  nail  consists  of  all  possible  frequencies  of  vibra- 
tion. It  is  similar  to  what  we  should  obtain  in  sound  if 
we  took  a  long  rod  of  wood  and  struck  all  the  keys  of  a 
piano  at  once.  The  light,  however,  which  we  get  from  a 
glowing  gas,  such  light  as  comes  from  the  long  glass  tubes 
now  frequently  used  in  garages  and  known  as  Cooper 
Hewitt  Lamps,  is  not  of  this  chaotic  composition.  It  can 
be  shown  to  consist  of  "  a  few  pure  notes,"  to  use  the  same 
acoustical  analogy.  These  pure  "  notes,"  or  colors,  are 
superimposed  upon  each  other  and  hence  the  combina- 
tion is  a  close  analogy  to  what  we  call  in  acoustics  a 
musical  chord. 

The  definite  "light  notes"  emitted  by  one  substance 
when  in  the  gaseous  form  are  perfectly  characteristic  of 
that  particular  species  of  atom  and  no  other  kind  of  atom 
gives  of  the  same  notes;  no  other  kind  of  atom,  that  is, 
emits  light  of  just  the  same  frequencies. 

This  can  only  mean  that  the  electrons  within,  or  about, 
one  kind  of  atom  are  in  some  definite  arrangement  or 
move  in  some  definite  way  and  one  which  differs  mark- 
edly from  that  to  be  found  in  atoms  of  other  species  (53). 


CHAPTER    X 

RECENT  DISCOVERIES  CONCERNING  ATOMIC 
STRUCTURE  AND  RADIATION 

The  last  five  years  have  brought  marked  developments 
in  fundamental  physical  theory. 

Of  the  three  fundamental  entities,  atom,  electron  and 
radiant  energy,  the  electron  alone  has  remained  to  our 
view  about  where  it  was  a  half  decade  ago,  but  our  con- 
ceptions of  the  atom  and  of  radiant  energy  have  changed 
considerably. 

[43] 


RECENT  DISCOVERIES  [Chap.  X 

Recent  Advances  Concerning  the  Atom.  —  New  light  on 
the  structure  of  the  atom  has  come  largely  through  the 
study  of  radio-active  substances.  From  researches  of 
Rutherford  and  his  associates,  it  now  seems  highly  prob- 
able that  the  atom  is  made  up  of  a  minute,  positively 
charged  nucleus  surrounded  by  several  rings  or,  better, 
regions  of  electrons.  The  total  number  of  electrons  is 
such  that  their  total  negative  charge  is  equal  to  the  posi- 
tive charge  on  the  nucleus. 

Since  the  volume  of  the  nucleus  and  the  volume  of  the 
electrons  is  in  all  cases  very  small  compared  with  the  di- 
mensions of  the  system,  the  major  part  of  the  volume  of 
an  atom  is  unoccupied  in  the  ordinary  sense  of  the  word. 
This  space  is  " empty"  in  the  same  sense  that  the  space 
around  a  large  charged  body  is  empty,  and  the  law  of 
electric  attraction  and  repulsion  within  the  atom  appears 
to  be  the  same  as  for  large  charges;  namely:  the  well- 
known  "  inverse  square  law." 

What  are  at  present  believed  to  be  the  facts  regarding 
atomic  structure  are,  perhaps,  somewhat  intricate,  and 
clearness  can  best  be  attained  by  making  a  formal  list. 

1.  The  atom  consists  of  nucleus  and  surrounding  electrons. 

2.  The  electrons  are  negatively  charged;    the  nucleus  posi- 
tively charged. 

3.  Both  electrons  and  nucleus  are  very  small  compared  to  the 
distances  between  them. 

4.  The  nucleus  is  composed  of  a  certain  amount  of  negative 
electricity  and  a  larger  total  amount  of  positive  electricity. 

5.  The  nucleus  may  lose  either  negative  or  positive  electric- 
ity, but  it  always  does  so  in  definite  units,  the  negative  unit  being 
the  electron  and  the  positive  unit  equal  to  two  electrons.    Such 
losses  occur  only  during  radio-activity.1 

6.  The  law  of  electric  attraction  between  the  nucleus  and  elec- 
trons in  the  surrounding  space  is  the  familiar  "inverse  square  law." 

1  Nothing  is  known  as  yet  regarding  actions  or  relations  inside 
the  nucleus. 

[44] 


Chap.  X]          STRUCTURE   OF  THE  ATOM 

7.  The  surrounding  electrons  have  a  total  charge  equal  to  the 
positive  charge  on  the  nucleus,  so  that  an  atom  in  the  ordinary 
state  has  no  resultant  charge. 

8.  The  electrons  surrounding  the  nucleus  exist  in  two  or  more 
rings  or  regions  at  markedly  different  distances  from  the  nucleus. 

9.  The  more  distant  electrons  are  those  chiefly  concerned  in 
chemical  action. 

10.  The  chemical  properties  of  an  element  are  determined  chiefly 
or  entirely  by  the  number  of  electrons  surrounding  the  nucleus 
and  hence  by  the  charge  on  the  nucleus.     (See  7  above.) 

11.  If,  therefore,  a  nucleus  loses  two  negative  units  (electrons) 
and  one  positive  unit  (see  5)  the  atom  which  results  will  not  be 
distinguishable  chemically  from  the  original  one. 

12.  The  above  action  will,  however,  result  in  a  loss  of  weight 
and  the  new  atom  will,  therefore,  have  a  less  atomic  weight  than 
the  original,  although  chemists  will  have  given  the  two  elements 
the  same  name.1 

13.  The   electrons   surrounding  the  nucleus   are   probably  in 
motion  about  it. 

14.  Violent  physical  phenomena  may  temporarily  remove  elec- 
trons from  the  atom  (but  not  from  the  nucleus)  and,  in  chemical 
combination,  one  atom  may  have  some  of  the  electrons  belonging 
to  another. 

15.  During  the  change  in  grouping  or  during  the  removal  of 
electrons  from  the  region  surrounding  the  nucleus,  electric  waves 
(light,  X  Rays)  are  radiated. 

16.  Changes  in  the  nucleus  only  occur  during  the  process  of 
radio-activity  and  cannot  therefore  in  any  way  be  caused  to  happen 
artificially. 

17.  The  " beta  rays"  are  electrons  escaping  from  the  nucleus 
and  the  " alpha  rays"  are  positive  units  escaping  from  it. 

18.  The  "gamma  rays"  are  the  electric  wave  radiations  result- 
ing when  electrons  are  violently  ejected  from  the  nucleus  during 
radio-activity. 

19.  There  is  some  mysterious  action  at  work  which  prevents 
the   surrounding   electrons   from   falling  into   the   nucleus.    This 
action  is  probably  closely  related  with  the  existence  of  the  "  Quanta  " 
of  Planck  (vide  infra). 

1  Two  such  chemically  identical  elements  with  different  atomic 
weights  are  now  called  "  isotopes." 

[45] 


RECENT  DISCOVERIES  [Chap.  X 

Atomic  Numbers.  —  Since  the  chemical  properties  of 
an  atom  seem  to  be  determined  principally,  if  not  entirely, 
by  the  resultant  charge  of  the  nucleus,  it  is  this  charge 
which  fixes  the  place  of  the  atom  in  the  "  Periodic  Table," 
so  familiar  to  chemists  (see  Section  6).  A  series  of  num- 
bers have  therefore  been  defined,  and  called  the  "  atomic 
numbers,"  which  appear  to  be  more  vital  to  the  chemist 
than  the  atomic  weight.  The  reason  for  this,  as  men- 
tioned before  (see  11  and  12,  above),  is  that  the  weight 
of  the  nucleus  depends  on  its  internal  make-up,  while 
its  action  on  the  surrounding  electrons  depends  only 
on  its  resultant  charge,  that  is,  only  on  the  excess  of 
positive  over  negative  electricity  present.  Weight  is  not 
an  interesting  chemical  feature ;  resultant  charge  (meas- 
ured by  the  atomic  number)  is. 

The  "Quantum"  Theory. —  A  decade  ago  an  epoch- 
making  paper  was  published  by  Max  Planck,  Professor 
of  Physics  in  the  University  of  Berlin.  The  revolutionary 
feature  in  Planck's  work  was  the  suggestion  that  the 
atoms  of  matter  did  not  radiate  energy  continuously,  but 
in  small  definite  units.  In  the  more  radical  forms  of 
the  theory,  as  developed  by  other  investigators,  radiant 
energy,  such  as  energy  from  the  sun,  is  considered  to 
be  in  the  form  of  separate  units  in  space:  "bullets  of 
energy,"  one  might  say.  This  means  that  the  light  and 
heat  from  the  sun  fall  on  the  earth  like  a  shower  of  rain. 
Planck  called  the  separate  units  "  Light  Quantities."  l 

This  conception  of  radiation  is  in  marked  contrast  to 
the  classical  wave-motion  theory  which  unquestionably 
represents  a  vast  host  of  facts  almost  ideally.    Accord- 
ing to  the  wave-motion  view,  a  disturbed  electron  radiates 
a  continuous  wave  in  all  directions  like  a  stone  dropped 
in  a  pond.    If  the  electron  made  a  regular  series  of  vibra- 
*  "Licht  Quanta." 
[46] 


Chap.  X]  THE    QUANTUM    THEORY 

tions,  a  train  of  such  waves  would  spread  out  in  concentric 
spheres  and  we  should  have  continuous  light  radiation. 

In  Planck's  theory,  however,  it  is  more  as  if  the  electron 
emitted  bullets  of  radiation.  Just  how  this  occurs  he  does 
not  attempt  to  say. 

Of  course  a  mere  baseless  suggestion  is  worth  nothing 
in  physics  and  Planck  would  not  have  hinted  at  such  a 
heterodox  notion  had  he  not  found  that  a  hitherto  stub- 
born paradox  in  the  theory  of  radiation  was  apparently 
removed  by  it. 

It  will  be  remembered  that  all  bodies  by  virtue  of  their 
temperature  send  out  radiant  energy.  In  general  the 
higher  the  temperature  the  more  total  energy  is  radiated 
in  a  second  and  also  the  higher  is  the  average  frequency 
(39)  of  the  radiation.  In  the  case  of  light  this  is  equivalent 
to  saying  that  the  higher  the  temperature  the  brighter 
and  the  bluer  is  the  emitted  light. 

Now  with  certain  limitations  the  amount  of  this  radia- 
tion had  been  found  by  experiment,  but  up  to  the  time  of 
Planck's  work  no  one  had  been  able  to  give  a  satisfactory 
theoretical  explanation  of  the  results.  The  two  most 
notable  attempts  at  explanation  were  those  of  Raylei^h 
and  Wien.  Each  of  these  covered  satisfactorily  part  of 
the  facts  but  neither  covered  all.  Planck  found  that  by 
using  the  new  hypothesis  he  could  not  only  give  an  ex- 
planation of  the  experimental  results,  but  his  mathemat- 
ical formula  contained  within  it  both  the  Rayleigh  and 
the  Wien  formulae. 

It  was  this  success  which  gave  Planck  the  courage  to 
make  such  an  extremely  heterodox  suggestion  as  that  of 
light  radiation  in  unit.:;. 

Although  meeting  with  this  initial  success  it  has  only 
been  within  the  last  few  years  that  Planck's  view  has,  in 
the  minds  of  those  most  competent  to  judge,  passed  from 

[47] 


RECENT  DISCOVERIES  [Chap.  X 

a  possibility  to  a  probability.  This  has  been  due  not  only 
to  further  study  of  the  direct  problem  of  radiation,  but  to 
indirect  evidence  received  from  other  fields  of  physics. 
Perhaps  the  most  important  confirmation  of  the  new 
view  has  come  from  a  study  of  the  action  of  light  on 
metals.  Certain  experiments  along  this  line,  notably 
those  of  Professor  Millikan  of  the  University  of  Chicago, 
give  results  which  in  the  opinion  of  some  are  almost  con- 
clusive evidence  for  the  existence  of  "Quanta." 

Less  direct,  but  also  pointing  hi  the  same  direction  are 
the  results  of  recent  very  low  temperature  research. 

The  " Quantum  Hypothesis"  has  certainly  become  a 
very  impressive  one,  to  say  the  least.  Just  how  impress- 
ive, will  of  course  depend  hi  part  on  the  temperament 
and  a  priori  ideas  of  the  investigator. 

As  to  a  priori  probability  it  should  be  noticed  that  at 
one  time  matter  and  electricity  were  both  pictured  as 
continuous  but  that  further  knowledge  showed  that  they 
were  both  atomic.  It  does  not  seem  very  strange  that 
the  third  member  of  the  trio  of  physical  fundamentals, 
namely  radiant  energy,  should  also  turn  out  to  be 
"atomic." 

The  real  difficulty  before  the  Quantum  Hypothesis  is 
the  necessity  of  joining  hands  with  the  classical  electrical 
theory.  There  can  at  present  be  no  reasonable  doubt 
that  the  oscillation  of  large  charges,  as  in  wireless  teleg- 
raphy, sends  out  long  waves  in  the  manner  required  by 
classical  theory.  Observation  and  experiment  are  unani- 
mous in  their  approval  of  the  well-known  electrical  laws. 
It  is  only  when  we  get  to  the  minute  charges  in  a  single 
atom  that  new  laws  seem  to  make  their  appearance. 

Perhaps  the  most  important  question  which  the  theory 
of  radiation  presents  to-day  is  therefore  the  following. 
How  is  it  possible  for  big  slow  phenomena  to  follow 

[48] 


Chap.  X]  NATURE   OF   X  RAYS 

one  set  of  laws  and  for  small  quick  phenomena  which 
are  unquestionably  of  the  same  general  nature,  to  follow 
another  set? 

The  answer  to  this  cannot  now  be  given,  but  it  seems 
probable  that  the  answer  when  it  comes  will  in  some  way 
show  that  one  set  of  laws  corresponds  to  the  "  averaging 
out"  of  the  other  set.  The  way  in  which  an  army  moves, 
as  seen  from  a  high  balloon,  is  quite  different  from  the 
way  a  single  man  moves,  and  yet  it  is  made  up  of  men. 
The  army  moves  continuously,  but  a  single  man  moves 
jerkily  in  steps. 

The  Quantum  Hypothesis  is  at  the  very  center  of  dis- 
cussion and  controversy  in  present-day  physics. 

The  Similarity  of  All  Forms  of  Radiant  Energy;  X 
Rays.  —  Until  very  recently  it  had  never  been  found 
possible  to  reflect  X  rays  in  the  way  in  which  light  is  so 
easily  reflected  from  a  mirror. 

It  had  been  suggested  by  several  investigators  that  this 
was  probably  due  to  the  fact  that  X  rays  were  waves  of 
such  extremely  short  length  that  the  flattest  surface  that 
could  be  made  was  too  rough  to  reflect  them.  If  any 
surface,  even  a  polished  one,  be  examined  with  a  high- 
power  microscope,  it  will  be  found  to  be  rough  and  ir- 
regular in  detail,  and  the  idea  was  that  such  a  surface, 
although  flat  for  light  waves,  might  be  like  a  ploughed 
field  for  the  extremely  more  minute  X  rays. 

The  obtaining  of  a  theoretically  " smooth"  surface  for 
X  rays  appeared  hopeless,  therefore,  until  Laue  sug- 
gested in  1910  that  natural  crystals,  if  carefully  broken, 
should  present  a  surface  smooth  and  flat  beyond  any- 
thing artificially  possible.  This  would  be  due  to  the  fact 
that,  hi  a  freshly  broken  crystal  surface,  the  molecules 
would  be  arranged  regularly,  like  the  bricks  in  a  pave- 
ment. 

[49] 


ATOMS  AND   LIFE  [Chap.  XI 

As  a  matter  of  fact,  when  a  crystal  of  mica  was  so  broken 
and  tested,  beautiful  X  ray  reflection  resulted.  This 
simple,  but  epoch-making  experiment,  served  as  the 
starting  point  of  a  long  series  of  experiments  by  various 
investigators  which  have  resulted  in  practically  proving 
that  X  rays  are  precisely  similar  to  light,  but  have  a 
shorter  wave-length. 

Just  as  X  rays  may  now  be  looked  upon  as  light  waves 
of  extremely  short  wave-length,  so  the  gamma  rays  which 
come  from  radio-active  substances  may  be  regarded  as  X 
rays  of  exceedingly  short  wave-length.  At  the  present 
time,  therefore,  it  is  very  probable  that  all  forms  of 
radiation,  from  Hertz  waves  through  "heat,"  light,  ultra- 
violet light,  and  X  rays,  to  gamma  rays  are  essentially 
similar  in  form,  though  how  they  can  at  once  show  wave 
characteristics  and  the  bullet  characteristics  of  the  Quan- 
tum view  is  still  a  puzzle. 


CHAPTER    XI 
ATOMS  AND  LIFE 

To  avoid  possible  objections  to  the  general  validity  of 
the  modern  theory  of  matter  it  may  be  well  to  point  out 
the  relation  of  atomic  ideas  to  the  physical  structure  of 
living  things.  Biologists  say  that  all  living  things  are 
composed  of  cells,  minute  objects  which  are  visible  in 
the  microscope  and  have  a  definite  structure.  Typically, 
then*  shape  is  not  very  distant  from  that  of  a  sphere,  and 
the  so-called  "cell-wall,"  like  the  skin  of  an  orange, 
simply  surrounds  the  whole.  These  cells,  they  say,  are 
singularly  independent,  sometimes  even  capable  of  living 
alone,  in  which  case  the  single  cell  is  called  a  "one-celled 
animal"  (or  plant). 

[50] 


Chap.  XI]  ATOMS  AND   THE   CELL 

From  a  modern  point  of  view  the  cell  is  thus  the  "vital 
unit"  of  all  living  things.  It  performs  for  itself  all  the 
life  functions  which  the  vast  aggregates  of  cells  which  we 
know  as  animals  or  plants  also  do. 

Now  it  is  clear  that  if  we  are  to  consider  all  matter  to 
have  atomic  structure,  living  substance  as  well  as  "dead," 
the  atom  must  be  very  small  indeed  compared  with  the 
cell,  for  otherwise  there  would  be  too  few  possibilities  of 
structure  to  account  for  the  wonderful  complexity  of  cell 
action.  A  rich  mosaic  cannot  be  made  from  a  small 
number  of  pieces. 

The  complexity  of  cell  action  and  that  of  living  things 
in  general  does  not,  however,  in  any  way  contradict  the 
ideas  of  atomic  structure  here  outlined,  for  the  reason 
that  most  cells  are  so  large  as  to  be  seen  clearly  with  the 
microscope.  Thus  the  smallest  cell  so  far  studied  with 
the  microscope  certainly  contains  several  million  atoms 
at  least,  and  hence  the  "mosaic  possibilities"  are  almost 
unUmited. 

From  a  fundamental  point  of  view  we  must  look  upon 
the  physical  side  of  the  changes  known  as  life  processes 
as  consisting  in  the  continual  rearrangement,  grouping 
and  regrouping,  of  the  molecules  themselves  or  of  the 
atoms  making  up  the  molecules.  With  so  many  atoms 
composing  the  physical  substance  of  a  single  cell  the 
possibilities  of  change  are  vast  (55). 


END  OF  PART  ONE 


[61] 


PART  II 

Section  1 

THE  SOURCES  OF  THE  MODERN  THEORY  OF 
MATTER 

The  foundation  of  the  "new  physics"  lies  in  the  work 
of  Michael  Faraday  upon  electro-magnetism  and  the  con- 
duction of  electricity  in  solutions,  as  well  as  in  the  sub- 
sequent development  of  his  ideas  by  J.  Clerk  Maxwell. 
Its  corner-stone  was  laid  by  J.  J.  Thomson  when,  hi  1898, 
he  discovered  the  electron.  The  investigations  of  Maxwell 
had  shown  that  light  is  probably  an  electrical  wave-motion, 
and  this  conception  was  strongly  confirmed  by  the  dis- 
covery of  the  long  electrical  waves  by  H.  R.  Hertz  in  1887. 
The  idea  of  "a  molecule  of  electricity"  was  clearly  sug- 
gested by  Maxwell,  and  the  name  " electron"  was  ap- 
plied to  the  conception  as  early  as  1881  by  G.  J.  Stoney. 
The  definite  physical  demonstration  of  its  reality  dates 
from  the  more  recent  work  of  Thomson,  P.  A.  Lenard, 
H.  A.  Lorentz,  and  others,  who  have  shown  that  electrons 
in  then*  vibrations  undoubtedly  generate  light. 

Another  line  of  discovery  which  has  been  of  tremendous 
importance  to  the  modern  theory  lies  hi  the  field  of  radio- 
activity, which  was  opened  by  the  investigations  of  Henri 
Becquerel  in  1896.  The  conception  of  the  atom  as  the 
indestructible  basis  of  chemical  change  was  first  system- 
atically defended  by  John  Dalton  in  1808.  The  science  of 
radio-activity  has  not  only  provided  us  with  a  new  source 
of  electrons,  but  it  has  led  us  to  the  conclusion  that  the 

NOTE :  The  Section  numbers  coincide  with  the  full-face  index 
numbers  in  the  text  of  Part  I. 

[52] 


Sec.  2]  HISTORY   OF  THE  THEORY 

chemical  atom,  although  a  fundamental  reality,  is  never- 
theless destructible.  Of  late  years  the  most  important 
work  in  this  new  science  has  been  done  by  Ernest  Ruther- 
ford, whose  treatise  on  the  subject  is  now  standard. 

The  discovery  of  "X  Rays"  by  W.  C.  Roentgen  in 
1895  was  another  important  event  in  the  development  of 
the  modern  conception  of  the  constitution  of  matter. 
The  problem  as  to  their  exact  nature  has  only  recently 
been  settled,  but  with  its  solution  have  come  remarkable 
advances  in  our  knowledge  of  the  structure  of  crystalline 
bodies. 

During  the  past  fifteen  years  a  host  of  new  workers  have 
appeared  and  progress  has  been  proportionately  rapid. 
At  the  present  time  we  seem  to  be  on  the  verge  of  epoch- 
making  discoveries  with  regard  to  the  nature  and  laws  of 
radiant  energy  (or  light),  which  may  bring  about  as  funda- 
mental a  change  in  the  ideas  of  ultimate  physics  as  did 
the  discovery  of  the  electron  and  of  the  phenomena  which 
it  underlies.  Besides  this  —  the  most  recent  develop- 
ment of  all  —  in  the  theory  of  "isotopism"  and  "  atomic 
numbers"  new  light  is  being  thrown  on  the  fundamental 
mystery  of  the  periodic  table  of  the  chemical  elements. 

REFERENCES 

An  ably  written  sketch  of  the  history  of  electricity  by  J.  A.  Flem- 
ming  is  to  be  found  under  " Electricity"  in  the  llth  edition  of  the 
Encyclopaedia  Britannica.  The  article  "Atom,"  in  the  same  work, 
may  also  be  consulted  to  advantage  in  connection  with  the  history 
of  the  theory  of  matter. 

Section  2 
METHODS   OF  DETERMINING  ATOMIC   SIZES 

There  are  a  great  many  possible  ways  of  determining 
the  approximate  size  of  atoms  and  molecules,  but  those 
which  are  the  simplest  and  most  direct  are  unfortunately 
the  least  accurate. 

[53] 


SIZES   OF   ATOMS  [Sec.  2 

Thin  Films.  —  It  is  obvious  that  if  the  atoms  have  a 
permanent  shape  and  volume  they  must  be  at  least  as 
small  in  diameter  as  the  thinnest  known  films  of  matter 
are  thick.  When  gold  is  flattened  by  the  process  of  gold- 
beating  the  atoms  are  spread  out  over  a  surface  greater 
than  that  which  they  previously  covered.  Now  a  sheet 
of  the  thinnest  gold-leaf  measures  about  one  one-mil- 
lionth of  an  inch  in  thickness  so  that  the  atoms  of  gold 
-which  are  among  the  largest  known  —  must  have  a 
diameter  at  least  as  small  as  one  one-millionth  of  an  inch. 
However,  we  are  familiar  with  liquid  films  which  are  much 
thinner  than  gold-leaf.  The  walls  of  soap-bubbles  and 
the  films  produced  by  pouring  oil  in  very  small  quan- 
tities upon  water  are  sometimes  as  thin  as  one  twenty- 
millionth  of  an  inch,  and  consequently  the  diameter  of 
the  atoms  which  compose  them  cannot  be  greater  than 
this  figure. 

Indirect  Methods.  —  To  arrive  at  more  exact  estimates 
of  the  size  of  atoms  it  is  necessary  to  resort  to  calcula- 
tion. Such  a  method  of  course  involves  some  assump- 
tions, but  there  are  many  different  ways  of  calculating 
atomic  sizes  in  which  nearly  as  many  different  assump- 
tions are  employed,  and  as  they  all  lead  to  substantially 
the  same  results  they  cannot  be  far  from  the  truth.  The 
facts  upon  which  calculations  of  this  sort  have  been  based 
vary  from  the  resistance  offered  to  the  passage  of  the 
electric  current  through  metals  and  liquids,  to  the  blue 
color  of  the  sky. 

The  Number  of  Atoms  in  a  Given  Volume. —  It  is  clear 
that  if  we  were  acquainted  with  the  number  of  atoms 
contained  in  a  given  body  we  could  specify  an  upper  limit 
for  the  size  of  a  single  atom,  by  dividing  the  volume  of  the 
body  as  a  whole  by  the  number  of  atoms  which  it  contained. 
Of  course  this  calculation  would  not  provide  us  with  an 

[64] 


Sec.  2]  NUMBER   OF   ATOMS 

exact  measure  of  the  atoms  themselves,  because  all  of  the 
space  in  the  body  is  not  occupied  by  the  atoms.  This 
is  due  in  part  to  the  fact  that  the  atoms  are  separated  under 
the  influence  of  the  constant  vibratory  motion  which  goes 
on  among  them,  but  it  would  also  be  true  even  if  they 
were  entirely  quiescent,  which  would  be  the  case  at  the 
so-called  "absolute  zero"  of  temperature,  as  explained 
further  on  in  Part  I.  However,  the  atoms  alone  could 
not  be  larger  than  the  size  determined  by  such  a  calcula- 
tion, and  if  the  data  upon  which  it  was  based  were  taken 
from  a  solid  substance  at  or  near  absolute  zero  it  would 
come  very  close  to  an  accurate  measure  of  the  actual 
atomic  size. 

Now  it  happens  that  it  is  relatively  easy  to  discover 
the  number  of  atoms  which  are  contained  in  a  given 
weight  of  almost  any  substance.  One  very  simple  and 
accurate  method  of  doing  this  is  to  weigh  the  amount  of 
the  substance  which  is  deposited  out  of  solution  by  the 
passage  of  a  certain  quantity  of  electricity.  Each  atomic 
particle  thus  deposited  carries  with  it  a  definite  and  known 
quantity  of  electricity,  the  natural  unit  of  electricity  or 
some  simple  multiple  of  this  unit.1  The  number  of  times 
this  unit  is  contained  in  the  quantity  of  electricity  which 
has  passed  gives  us  a  measure  of  the  number  of  atoms  in 
the  amount  of  substance  which  has  been  deposited  by 
the  current.  Since  this  substance  can  be  collected  and 
its  volume  determined,  we  are  able  to  specify  the  number 
of  atoms  contained  in  a  given  volume  of  the  substance 
in  question,  although  the  problem  is  slightly  more  complex 
than  we  have  represented  it. 

From  this  we  can  calculate  easily  the  upper  limit  of 
atomic  size  to  which  we  have  referred. 

1  Which  happens  to  be  the  charge  borne  by  the  electron,  and 
which  can  be  determined  by  other  means. 

[55] 


SIZES   OF   ATOMS  [Sec.  2 

Recently  it  has  been  found  possible  to  count  individu- 
ally the  helium  atoms  (see  Section  48),  which  are  shot 
off  from  radio-active  substances,  so  that  after  a  measur- 
able amount  of  this  gas  has  collected,  the  number  of  atoms 
(77  billion  billion  per  cubic  inch)  of  which  it  is  made  up 
is  accurately  known. 

Other  Methods:  the  Atoms  of  Gases.  —  Another  fairly 
direct  way  of  getting  the  magnitude  of  atoms  depends  upon 
a  measurement  of  the  speed  at  which  atoms  move  through 
a  liquid  or  a  gas  under  the  influence  of  a  known  force. 
The  smaller  the  particles  into  which  a  given  quantity  of 
matter  is  divided  the  more  slowly  it  will  move  through  a 
fluid  under  the  action  of  a  fixed  force.  Everybody  is 
familiar  with  this  fact  in  the  case  of  falling  dust  particles, 
which  are  all  acted  upon  by  one  force,  gravity.  The  larger 
ones  settle  very  rapidly,  whereas  the  small  particles  may 
remain  apparently  motionless  in  quiet  air  for  a  long  time. 
Under  the  right  conditions  atoms  can  be  made  to  move 
through  liquids  and  gases  under  forces  as  definitely 
known  as  that  of  gravity,  and  although  these  substances 
are  themselves  made  up  of  atoms  the  nature  of  the  resist- 
ance which  the  moving  atoms  encounter  is  probably  not 
very  different  from  that  which  would  be  offered  to  the 
passage  of  larger  bodies.  Of  course  we  cannot  measure 
the  rate  of  motion  of  single  atoms,  but  we  can  determine 
that  of  large  numbers  of  them,  which  is  equally  satisfac- 
tory to  our  purpose.  Knowing  the  speed  of  the  travel- 
ling atoms  and  the  magnitude  of  the  forces  which  are 
propelling  them,  we  can  easily  calculate  their  diameters, 
which  according  to  this  method  turn  out  to  be  about  one 
one-hundred-millionth  of  an  inch. 

Still  more  accurate  methods  are  known  for  the  calcu- 
lation of  atomic  sizes,  but  most  of  these  are  dependent 
upon  complex  considerations  into  which  we  cannot  enter 

[56] 


Sec.  2]  ATOMS   OF   GASES 

here.  For  example,  the  size  of  the  atoms  of  a  gas  can  be 
calculated  from  the  rate  at  which  heat  is  conducted 
through  the  body  of  the  gas.  Heat  consists  in  the  rapid 
random  motion  of  the  gas  molecules,  and  it  is  clear  that 
the  larger  these  molecules  are,  the  more  they  will  impede 
each  other's  motion  and,  consequently,  the  more  slowly 
this  motion  will  be  transferred  from  one  part  of  the  gas  to 
another.  But  such  a  transfer  is  identical  with  the  conduc- 
tion of  heat.  If  we  combine  our  knowledge  of  the  rate  of 
conduction  with  that  of  the  number  of  molecules  contained 
within  a  given  volume  of  the  gas  we  can  calculate  the  size 
of  the  molecules  themselves.  In  the  case  of  simple  sub- 
stances, molecular  and  atomic  sizes  are  of  the  same 
order  of  magnitude. 

Other  methods  of  figuring  atomic  sizes  depend  upon 
measurements  of  the  so-called  "viscosity"  of  gases,  or 
their  internal  friction.  It  is  also  possible  to  get  a  very 
exact  idea  of  the  diameter  of  the  gas  molecules  by  study- 
ing the  deviations  from  the  well-known  "law  of  Boyle" 
(see  Section  18  below),  which  states  that  the  volume  of  a 
gas  is  inversely  proportional  to  the  pressure  under  which 
it  is  confined.  These  deviations  are  due  to  the  influence 
of  the  volume  of  the  atoms  themselves  upon  that  of  the 
gas  as  a  whole.  The  atoms  in  most  gases  are  very  far 
apart  compared  with  their  sizes,  but  as  the  gas  is  com- 
pressed—  that  is,  as  the  atoms  are  forced  nearer  and 
nearer  together  —  the  atomic  volume  begins  to  make 
itself  felt,  and  this  fact  provides  us  with  a  basis  upon 
which  to  calculate  that  volume.  Another  well-known 
method  is  founded  upon  the  rates  at  which  gases  "dif- 
fuse" into  each  other,  and  modern  studies  in  electricity 
have  yielded  other  very  exact  means  of  determining  the 
diameters  of  atoms.  The  new  science  of  radio-activity 
has  also  contributed  to  our  knowledge  of  atomic  sizes  by 

[57] 


VISIBILITY   OF  ATOMS  [Sec.  3 

furnishing  the  experimenter  with  individual  atoms  bear- 
ing electrical  charges  to  make  them  conspicuous,  and 
moving  at  almost  inconceivably  high  velocities. 

Agreement  of  Results.  —  The  results  of  these  diverse 
methods  of  discovering  the  magnitude  of  atoms  are  in  a 
harmony  so  substantial  that  it  is  practically  impossible 
to  doubt  their  truth.  The  diameter  of  the  smallest  atoms 
must  be  closely  in  the  neighborhood  of  one  three-hun- 
dred-millionth of  an  inch,  and  that  of  the  largest  cannot 
be  many  times  this.  What  may  perhaps  be  called  "the 
realm  of  atomic  magnitudes  "  is  accordingly  very  definite 
and  limited. 

REFERENCES 

For  a  more  detailed  discussion  of  the  means  by  which  the  sizes 
of  atoms  and  molecules  are  calculated  the  reader  may  consult: 

A.  D.  Risteen's  "Molecules  and  the  Molecular  Theory,"  1895, 
pp.  133-151. 

A  list  of  specific  molecular  diameters,  calculated  from  modern 
measurements  is  given  by  W.  Sutherland  in  the  English  Philo- 
sophical Magazine  for  February,  1908,  vol.  17,  pp.  320-321.  A  very 
simple  discussion  of  atomic  sizes  will  be  found  in  A.  E.  Dolbear's 
"Matter,  Ether,  and  Motion,"  enlarged  edition,  1894,  pp.  8-26. 

On  molecular  volumes  and  their  calculation,  see  W.  Nernst's 
"Theoretical  Chemistry,"  (Eng.  Trans.),  pp.  304-307. 

Section  3 
ATOMS,  COLLOIDS,   AND  THE  MICROSCOPE 

To  make  a  single  atom  visible  to  the  eye  we  should 
require  a  microscope  capable  of  enlarging  the  apparent 
size  of  objects  nearly  a  million  times.  The  so-called 
" ultra-microscope"  has  a  power  closely  approaching  this, 
and  still  more  powerful  instruments  might  be  built  if 
optical  principles  did  not  interfere.  Objects  very  much 
smaller  than  light  waves  will  not  reflect  light  in  the  usual 
way,  /.c.,  so  that  their  surfaces  can  be  seen.  However, 
they  are  often  detectable  under  the  ultra-microscope  as 

[68] 


Sec.  3]  COLLOIDS 

minute  luminous  specks  or  points.  Particles  having  a 
diameter  of  about  three  millionths  of  an  inch  have  been 
detected  by  means  of  the  ultra-microscope.  This  is 
approximately  two  hundred  times  the  diameter  of  the 
average  atom.  Whether  or  not  we  shall  ever  be  able  to 
prove  the  existence  of  atoms  and  molecules  by  sight  is  a 
question  which  must  be  decided  by  the  progress  of  opti- 
cal invention,  but  it  is  well  to  bear  in  mind  the  fact  that 
even  now  the  limits  of  visibility  lie  not  very  far  from  the 
realm  of  atomic  sizes. 

The  particles  seen  under  the  ultra-microscope  are 
those  characteristic  of  so-called  colloidal  substances. 
There  is  a  continuous  gradation  of  sizes  among  such 
particles,  from  those  of  strictly  molecular  magnitude  up 
to  the  very  much  larger  granules  which  are  found  in  an 
"emulsion"  or  a  "suspension."  There  is  thus  no 
sharp  line  dividing  colloidal  particles  from  large  mole- 
cules but,  nevertheless,  colloidal  substances  have  re- 
markable properties  which  distinguish  them  from  strictly 
molecular,  or  "crystalloid"  bodies.  These  properties 
are  exhibited  most  distinctly  in  aqueous  solution  or  sus- 
pension, and  include  such  phenomena  as  coagulation 
and  gelatination.  Most  of  the  substances  characteristic 
of  living  organisms  are  in  the  colloidal  state,  and  it  is 
almost  certain  that  life  would  be  impossible  without 
colloids.  However,  the  fundamental  peculiarity  about 
colloids,  is  simply  their  degree  of  subdivision,  or  "dis- 
persion," as  it  is  sometimes  called. 

REFERENCES 

On  the  use  and  results  of  the  ultra-microscope  see  R.  Zsigmondy's 
"Colloids  and  the  Ultra-Microscope"  (Eng.  Trans.),  1909,  esp. 
page  122  on  the  limits  of  visibility. 

For  a  popular  discussion  of  the  questions  above  raised  see  A.  E. 
Dolbear's  "  Matter,  Ether,  and  Motion,"  enlarged  edition,  1894, 
pp.  8-26. 

[69] 


SHAPE   OF  ATOMS  [Sec.  4 

Section  4 
THE  SHAPE   OF  ATOMS 

Why  Atoms  are  Supposed  to  be  Spherical.  —  It  is  obvious 
that  the  shape  of  an  atom  or  molecule  will  materially 
affect  certain  of  the  properties  of  the  bodies  of  which  it 
forms  a  part.  For  instance,  if  the  molecules  of  gases  had 
the  form  of  long  cylinders  or  rods  all  of  the  properties  of 
the  gases  which  depend  upon  the  ease  of  movement  of 
the  molecules  among  themselves  would  be  different  from 
what  they  would  be  if  the  molecules  were  spherical. 
Indeed,  it  is  impossible  to  calculate  the  characteristic 
1  'constants"  of  gases  on  the  basis  of  the  molecular  theory 
without  the  use  of  some  definite  assumption  about  the 
shape  of  its  molecules.  The  one  usually  made  is  that 
the  molecules  are  spherical,  and  since  it  leads  to  general 
results  which  are  in  harmony  with  the  facts  it  must  be 
nearly  correct. 

When  a  number  of  bodies  are  thrown  together  in  a  heap 
the  size  of  the  heap  will  depend  not  only  upon  the  volume 
of  the  individual  bodies  and  upon  the  number  present, 
but  also  upon  their  shape.  Thus,  if  they  were  all  cubes 
and  were  packed  neatly  together  they  would  fill  the  whole 
space  marked  off  by  the  "heap."  However,  if  they  were 
spheres  they  could  not  possibly  be  arranged  so  as  to  take 
up  all  of  this  space. 

Now  we  have  a  number  of  means  of  estimating  the 
volume  of  the  individual  atoms  or  molecules  of  substances, 
which  are  independent  of  any  knowledge  of  the  volume 
occupied  by  large  masses  of  these  substances.  If  we  as- 
sume that  the  molecules  are  spherical,  therefore,  we  can 
calculate  the  amount  of  space  which  should  be  filled  by 
a  body  containing  a  given  number  of  them  under  certain 

[60] 


Sec.  4]  THE   SATURNIAN   ATOM 

conditions,  as  for  example  those  which  hold  at  absolute 
zero,  when  the  molecules  are  motionless.  We  can  then 
compare  this  volume  with  that  actually  occupied  under 
these  conditions  by  a  body  made  up  of  the  same  number 
of  molecules  of  the  special  substances  which  we  are 
studying.  If  the  two  are  essentially  the  same  we  shall 
have  some  reason  for  supposing  that  our  original  assump- 
tion concerning  the  roundness  of  the  molecules  or  atoms 
was  correct.  Calculations  of  this  sort  have  been  made 
and  indicate  that  most  atoms  are  either  spherical  or  are 
slightly  flattened  spheres. 

The  "  Saturn  fan  Atom."  •  —  Although  it  is  desirable  for 
purposes  of  clearness  to  think  of  the  atom  as  possessing 
a  definite  surface  having  a  definite  contour,  it  is  also  im- 
portant to  bear  in  mind  the  fact  that  the  resemblance 
between  an  atom  and  a  solid  ball  probably  amounts  to 
little  more  than  an  analogy.  As  represented  in  the  fig- 
ures in  the  text,  the  surfaces  or  "  edges  "  of  an  atom  are 
probably  ill  defined,  and  they  may  even  be  changeful. 
Certain  modern  considerations  —  which  are  touched  upon 
in  Section  53  —  make  it  seem  altogether  likely  that  the 
atom  is  really  a  system  of  rapidly  rotating  particles  which 
are  very  much  smaller  than  the  atom  itself.  If  this  theory 
should  prove  to  be  true  the  size  and  shape  of  the  atom 
would  merely  be  those  of  the  " orbits"  of  the  outermost 
of  these  rotating  particles,  just  as  the  size  and  shape  of 
the  solar  system  depends  upon  those  of  the  orbits  of  the 
outermost  planets. 

Some  of  the  most  recent  studies  on  the  shape  of  the 
atoms  have  been  made  by  R.  D.  Kleeman. 

REFERENCES 

Kleeman's  discussion  of  atomic  shapes  is  rather  difficult,  but 
can  be  found  in  the  Philosophical  Magazine  for  July,  1910,  vol.  20, 
pp.  229-238. 

[61] 


ATOMIC   SPECIES 


[Sec.  5 


As  an  illustration  of  the  manner  in  which  the  assumption  of 
the  sphericity  of  the  atom  or  molecule  enters  into  the  theory  of 
gases  see:  W.  P.  Boynton's  "Applications  of  the  Kinetic  Theory," 
(1904)  Chapter  IE. 

A  very  popular  discussion  of  "  Solar  system  "  conception  of  the 
atom  is  given  in  Chapter  IV  of  C.  R.  Gibson's  "Scientific  Ideas 
of  To-day,"  (1909)  pp.  52-58. 

Section  5 

SPECIES    OF   ATOMS;    ATOMIC  WEIGHTS,  AND   ATOMIC 
VOLUMES 

The  official  table  of  the  chemical  elements  now  con- 
tains eighty-three  names,  each  of  which  corresponds  with 
a  particular  species  of  atom.  In  addition  to  these  we  must 
include  in  our  list  of  atomic  species  the  many  unstable 
elements,  which  have  been  discovered  by  the  new  science 
of  radio-activity. 

The  first  table  below  gives  the  names  of  the  recognized 
chemical  elements  with  their  "  atomic  weights"  on  the 
basis  of  oxygen  =  16.  The  second  table  performs  the 
same  service  with  respect  to  the  new  radio-active  ele- 
ments, several  of  which  are  also  included  in  the  first 
table. 


TABLE  I.    THE  CHEMICAL  ELEMENTS 


Atomic       Name  of 
Number     Element 

1  Hydrogen . 

2  Helium 

3  Lithium  .  .  . 
Glucinium  . 

Boron 

Carbon  .  .  . 
Nitrogen.  .  . 
Oxygen 
Fluorine  . .  . 

Neon 

Sodium 

Magnesium 

Aluminium 

Silicon 

Phosphorus 
Sulphur  .  .  . 
Chlorine.    . 
Argon  


10 
11 
12 
13 
14 
15 
16 
17 
18 


Sym- 
bol 
..  H 

He 

Li 
.  .Gl 

B 

C 

N 

O 

F 

Ne 

Na 
.  .  Me 
.  .  .Al 

Si 
.  .  .P 

S 

Cl 

A 


Atomic 
Weight 

1.008 

3.99 

6.94 

9.1 
11.0 
12.00 
14.01 
16.00 
19.0 
20.2 
23.00 
24.32 
27.1 
28.3 
31.04 
32.07 
35.46 
39.88 


Nature  under  Ordinary  Date 

Conditions  Discovered 

Very  light  gas,  chemically  active.  .    1766 

Very  light  gas,  chemically  inert  .  .  . 

Light  alkali-forming  metal  ........ 

Light  metal,  resembling  zinc  ..... 

Non-metal,  occurs  in  "borax"  ..... 

Non-metal  (charcoal  and  diamond) 
Atmospheric  gas,  inert  ........... 

Atmospheric  gas,  chemically  active  . 
Excedingly  corrosive  gas  .........   1810 

Inert  atmospheric  gas  ............    1898 

Alkali  metal,  occurs  in  "salt  "    ____    1807 

White,  combustible  metal  ........    1808 

Light,  white  metal    ..............    1828 

Hard,  semi-metallic  substance  ____   1823 


1868 
1817 
1828 
1808 

P 

1772 
1774 


Inflammable  non-metal 
Inflammable  non-metal 
Yellow,  corrosive  gas 
Inert  atmospheric  gas 

[62] 


1738 

P 

1810 
1894 


Sec.  6] 


ATOMIC   WEIGHTS 


19  Potassium K     39.10  Alkali  metal,  occurs  in  "potash"  . . .  1807 

20  Calcium Ca   40.07  Alkaline  earth,  occurs  in  "lime"  .  .   1808 

21  Scandium Sc      44.1         Metal  found  in  rare  earths 1879 

22  Titanium Ti      48.1  Metal  found  in  rare  earths 1796 

23  Vanadium V       51.0  Metal  found  in  rare  earths 1801 

24  Chromium .  .  .  .  Cr      52.0  Hard  metal  forming  highly   colored 

compounds 1797 

25  Manganese .  .  .  Mn    54.93  Grayish  metal 1774 

26  Iron Fe      55.84  Grayish  metal P 

27  Cobalt Co     58.97  Silvery-white  metal 1733 

28  Nickel Ni      58.68  Hard,  silvery-white  metal 1751 

29  Copper Cu     63.57  Reddish-yellow  metal P 

30  Zinc Zn     65.37  Silvery-white  metal 1520 

31  Gallium Ga     69.9  Hard,  grayish-white  metal 1875 

32  Germanium. .  .Ge     72.5  Grayish,  metallic  solid 1886 

33  Arsenic As      74.96  Brittle,  steel-gray  semi-metallic  subs.  1649 

34  Selenium Se     79.2  Sulphur-like  solid 1817 

35  Bromine Br      79.92  Heavy,  corrosive,  red  liquid 1826 

36  Krypton Kr     82.92  Inert  atmospheric  gas 1897 

37  Rubidium Rb     85.45  Soft,  white,  alkali-metal 1868 

38  Strontium Sr      87.63  Alkaline-earth  metal 1808 

39  Yttrium Yt      89.0  Dark  gray  metal 1823 

40  Zirconium Zr      90.6  Hard,  brittle,  iron-gray  metal 1824 

41  Columbium .  .  .  Cb     93.5  Steel-gray  metal 1846 

42  Molybdenum  .Mo    96.0  Metal  resembling  iron 1782 

44  Ruthenium . . . .  Ru  101.7  Platinum-like  metal 1844 

45  Rhodium Rh  102.9  "Infusible,"  silvery  metal 1804 

46  Palladium Pd   106.7  Fusible,  platinum-like  metal 1803 

47  Silver Ag    107.88  Metal,  of  familar  properties P 

48  Cadmium Cd   112.40  White,  malleable,  zinc-like  metal  .  .  .   1817 

49  Indium In    114.8  Soft,  malleable,  zinc-like  metal 1863 

50  Tin Sn    119.0  Silver  white,  very  malleable  metal .  .      P 

51  Antimony Sb    120.2  Brittle,  white,  crystalline  metal 1450 

52  Tellurium Te    127.5  White,  semi-metallic  solid 1782 

53  Iodine I      126.92  Volatile,  brown,  non-metallic  solid.  .   1811 

54  Xenon Xe   130.2  Inert  atmospheric  gas 1898 

55  Caesium Cs    132.81  Soft  alkali-metal 1860 

56  Barium Ba    137.37  Soft  silver-white  alkali-earth  metal .   1808 

57  Lanthanum.  .  .La    139.0  Malleable  metal  found  in  rare  earths.  1841 

58  Cerium Ce    140.25  Rare-earth  metal  resembling  iron .  .  .   1801 

59  Praseodymium  Pr    140.6  Rare  earth  metal 1885 

60  Neodymium. .  .Nd  144.3  Rare  earth  metal 1885 

62  Samarium .  .  .  .  Sa     150.4  Rare  earth  metal 1879 

63  Europium Eu   152.0  Rare  earth  metal 1901 

64  Gadolinium.  .  .Gd  157.3  Rare  earth  metal 1886 

65  Terbium Tb   159.2  Rare  earth  metal 1843 

66  Dysprosium .  .  .Ds    162.5  Rare  earth  metal 1907 

67  Holmium Ho  163.5  Rare  earth  metal 1886 

68  Erbium Er    167.7  Rare  earth  metal 1843 

69  Thulium Tm  168.5  Rare  earth  metal 1879 

70  Ytterbium .  .  .  .  Yb   172.0  Rare  earth  metal 1878 

71  Lutecium Lu    174.0  Rare  earth  metal 1908 

73  Tantalum Ta    181.5  Hard,  silvery,  ductile  metal 1802 

74  Tungsten W    184.0  Brittle,  very  "  infusible,  "  metal 1783 

76  Osmium Os   190.9  Blue-gray  resistant  metal 1804 

77  Iridium Ir     193.1  Brittle,  "  infusible,"  metal 1804 

78  Platinum Pt    195.2  Silvery,  "  infusible,"  metal 1500 

79  Gold Au   197.2  Chemically  resistant  metal P 

80  Mercury Hg  200.6  Liquid,  silvery  metal 300  B.  C. 

81  Thallium Tl    204.0  Soft,  whitish  metal 1861 

82  Lead Pb  207.10  Soft,  bluish-white  metal P 

83  Bismuth Bi    208.0  Very  brittle  reddish-white  metal.  .  .  .   1450 

86  Niton Nt    222.4  Inert  gas  (radium  emanation) 1900 

88  Radium Ra  226.4  Radio-active  alkaline-earth  metal . .     1898 

90  Thorium Th  232.4  Heavy,  rare-earth  metal 1828 

92  Uranium Ur   238.5  Hard,  white,  malleable  metal 1780 

[63] 


ATOMIC   SPECIES  [Sec.  6 

The  atomic  weights  above  are  those  given  by  the 
"International  Committee  on  *  Atomic  Weights,'  for 
1914."  "P"  in  the  last  column  stands  for  "prehistoric." 

TABLE  H.      THE   RADIO-ELEMENTS. 
URANIUM-RADIUM   SERIES 

Atomic  Name  of  Atomic  Chemical 

Number  Element  Weight  Analogue 

92  Uranium  1 238.15 Tungsten  (h) 

90  Uranium  Xi 234 Thorium  (i) 

91  Uranium  X2    234 Tantalum  (h) 

92  Uranium  2 234 Uranium  1  (i) 

90  Ionium 230 Thorium  (i) 

88  Radium 225.95  (HBnigschmid)  ....   Barium  (h) 

86  Radium  Emanation  (Niton) . .  .222 Xenon  (h) 

84         Radium  A 218 Polonium  (i) 

82  Radium  B 214 Lead  (i) 

83  Radium  C 214 Bismuth  (i) 

84  Radium  C' 214 Polonium  (i) 

81  Radium  C2   210 Thallium  (i) 

82  Radium  D  (Radio-Lead) 210 Lead  (i) 

83  Radium  E 210 Bismuth  (i) 

84  Radium  F  (Polonium)  210 Tellurium  (h) 

82  Radium  G  (probably  Lead)  . . 207.10  (?) 

THORIUM   SERIES 

90         Thorium 232.4 Cerium  (h) 

88  Meso-Thorium  1 228 Radium  (i) 

89  Meso-Thorium  2 228 Actinium  (i) 

90  Radio-Thorium 228 Thorium  (i) 

88  Thorium  X 224 Radium  (i) 

86         Thorium  Emanation 220 Niton  (i) 

84         Thorium  A 216 Polonium  (i) 

82  Thorium  B 212 Lead  (i) 

83  Thorium  C  212 Bismuth  (i) 

84  Thorium  C' 212 Polonium  (i) 

81  Thorium  D 208 Thallium  (i) 

ACTINIUM   SERIES 

89  Actinium 230  (?) Lanthanum  (h) 

90  Radio-Actinium 230  (?) Thorium  (i) 

88        Actinium  X 226  (?) Radium  (i) 

86         Actinium  Emanation 222  (?) Niton  (i) 

84  Actinium  A 218  (?) Polonium  (i) 

82  Actinium  B 214  (?) Lead  (i) 

83  Actinium  C 214  (?) Bismuth  (i) 

81  Actinium  D 210  (?) Thallium  (i) 

Only  ten  of  the  thirty-five  substances  named  above 
are  believed  to  represent  chemically  new  atomic  species. 
The  atomic  weights  of  only  a  few  of  them  are  accurately 

[64] 


Sec.  6]  FINDING  ATOMIC   WEIGHTS 

known ;  those  of  the  others  are  estimated  on  the  basis  of 
the  theory  of  radio-active  disintegration.  The  last  column 
gives  the  name  of  another  element  to  which  the  given 
element  is  closely  similar.  In  the  cases  marked  (i)  the 
two  elements  are  "isotopic"  i.e.,  although  of  different 
atomic  weight  or  radio-activity,  are  identical  in  chemical 
character,  (h)  indicates  a  "homologue"  in  the  periodic 
table. 

The  weights  of  the  atoms  are  expressed  in  terms  of 
the  weight  of  the  lightest  atom,  namely  that  of  hydro- 
gen.1 Thus,  when  we  say  that  the  "atomic  weight"  of 
oxygen  is  16  we  mean  that  an  atom  of  oxygen  weighs 
16  times  as  much  as  an  atom  of  hydrogen. 

Method  of  Finding  Atomic  Weights.  —  The  methods  by 
which  the  relative  weights  of  the  atoms  are  determined 
are  not  difficult  to  understand.  Suppose,  for  example, 
that  the  chemist  desires  to  learn  the  atomic  weight  of  the 
element  chlorine.  He  knows  by  experiment  that  chlorine 
gas  will  combine  with  hydrogen  gas  in  definite  propor- 
tions to  form  hydrochloric  acid.  When  he  analyzes  this 
compound  he  finds  that  it  contains  35.18  parts  of  chlorine 
by  weight  to  one  part  of  hydrogen.  Consequently  on  the 
assumption  that  the  molecule  of  hydrochloric  acid  is  made 
up  of  one  atom  of  hydrogen  and  one  atom  of  chlorine,  he 
is  able  to  determine  the  atomic  weight  of  chlorine  as  35.18, 
assuming  that  of  hydrogen  to  be  unity  (or  as  35.46  on 
the  basis  of  the  weight  of  oxygen  taken  equal  to  16). 
Similar  measurements  can  be  made  of  the  proportions  by 
weight  in  which  other  elements  combine  with  hydrogen, 
and  these  will  furnish  a  basis  for  the  calculation  of  their 
atomic  weights  also. 

1  As  a  matter  of  fact,  the  unit  atomic  weight  now  generally  used 
is  TV  the  weight  of  the  oxygen  atom,  but  this  differs  only  slightly 
from  that  of  the  atom  of  hydrogen. 

[66] 


ATOMIC   SPECIES  [Sec.  6 

It  may  well  be  asked  how  it  is  that  the  chemist  can 
know  the  number  of  atoms  of  particular  elements  which 
are  contained  in  a  given  compound  unless  he  first  knows 
the  atomic  weights  of  these  elements.  For  example,  the 
compound,  water,  is  made  up  of  one  part  of  hydrogen  to 
eight  of  oxygen,  but  instead  of  stating  on  the  basis  of  this 
fact  that  the  atomic  weight  of  oxygen  is  8  the  chemist 
comes  to  the  conclusion  that  water  contains  two  atoms  of 
hydrogen,  so  that  the  atomic  weight  of  oxygen  is  16. 
One  of  the  things  which  leads  him  to  this  conclusion  is 
the  fact  that  if  he  takes  the  atomic  weight  of  oxygen  as  8, 
he  will  soon  find  it  necessary  to  suppose  that  the  mole- 
cules of  certain  substances  other  than  water,  contain 
oxygen  in  less  than  atomic  quantities,  which  is  impossible. 
Thus  by  comparing  many  different  results  derived  from 
the  analysis  of  different  compounds  containing  the  same 
elements,  the  chemist  is  led  finally  to  the  selection  of 
relative  weights  in  harmony  with  the  atomic  theory. 

However,  there  are  other  considerations  which  lead  to 
similar  results.  It  is  a  well-known  principle  that  equal 
volumes  of  all  gases  under  the  same  conditions  contain 
the  same  number  of  molecules,  and  from  this  fact  it  fol- 
lows that  if  we  weigh  equal  volumes  of  different  gases 
under  these  constant  conditions  we  will  obtain,  imme- 
diately, the  relative  weights  of  their  constituent  mole- 
cules. Thus,  when  water  is  broken  up  into  hydrogen  and 
oxygen  gas  it  yields  two  volumes  of  the  former  for  one  of 
the  latter,  a  fact  which  seems  to  indicate  that  twice  as 
many  atoms  of  hydrogen  are  present  in  a  molecule  of 
water  as  there  are  atoms  of  oxygen.  It  is  obvious  that  the 
" molecular  weights"  of  all  substances  which  are  capable 
of  being  converted  into  a  gas  can  be  found  by  measuring 
the  weight  of  a  unit  volume  of  this  gas  under  standard 
conditions.  Such  a  knowledge  of  the  relative  weights  of 

[66] 


Sec.  5]  ATOMIC   VOLUMES 

the  molecules  composing  a  compound  will  clearly  be 
of  the  utmost  value  in  the  determination  of  the  weights 
of  the  atoms  of  which  they  in  turn  are  composed. 

Some  of  the  elements  do  not  combine  with  hydrogen  to 
form  substances  which  it  is  convenient  to  analyze.  How- 
ever, the  atomic  weights  of  such  elements  may  be  found 
indirectly  by  studying  the  ratios  in  which  they  unite  with 
other  elements  of  known  atomic  weight.  Thus,  most 
elements  which  have  slight  affinity  for  hydrogen  combine 
very  readily  with  oxygen,  and  since  hydrogen  also  forms 
a  definite  compound  with  oxygen,  this  latter  element 
constitutes  a  connecting  link  between  hydrogen  and  the 
element  whose  atomic  weight  we  desire  to  determine. 

Atomic  Weights  and  Atomic  Volumes.  —  The  idea  has 
recently  been  advocated  by  J.  Traube  that  a  simple  and 
definite  relationship  exists  between  the  size  of  an  atom 
of  any  element  and  its  weight.  The  volume  of  the  atom 
appears  to  vary  approximately  as  the  square-root  of  its 
weight.  This  means  that  the  diameter  of  a  spherical  atom 
would  be  proportional  to  the  sixth  root  of  its  weight,  so 
that  although  atomic  diameters  vary  rather  widely  among 
the  lighter  elements  the  diameters  of  the  heavier  atoms 
are  very  nearly  alike.  This  law  connecting  atomic  weights 
and  volumes  is  based  upon  measurements  of  essentially 
the  same  character  as  those  which  we  have  discussed  in 
Section  2,  above.  However,  it  appears  also  to  be  in  har- 
mony with  certain  phenomena  in  radio-activity  which 
seem  to  depend  upon  the  volume  of  the  atoms  composing 
a  body. 

REFERENCES 

A  table  of  the  radioactive  substances,  with  their  properties,  will 
be  found  in  Kaye  and  Laby's  "  Tables  of  Physical  and  Chemical 
Constants"  (1911),  pp.  107-108.  A  more  elaborate  discussion  will 
be  found  in  Ernest  Rutherford's  "Radio-Active  Substances  and 
Their  Radiations,"  (1913).  For  the  most  recent  developments  see 

[67] 


THE  PERIODIC   TABLE  [Sec.  6 

the  1915  edition  of  Frederick  Soddy's  "The  Chemistry  of  the 
Radio-Elements,"  pp.  70-142. 

On  the  choice  of  atomic  weights,  see  Wilhelm  Ostwald's  "  Out- 
lines of  General  Chemistry"  (1890),  Part  I,  Book  VI,  Chap.  I, 
pp.  178-182. 

An  excellent  detailed  discussion  of  the  determination  of  atomic 
weights  also  appears  in  H.  C.  Jones'  "The  Elements  of  Physical 
Chemistry"  (1902),  pp.  4-18. 


Section  6 
THE  PERIODIC  TABLE  OF  THE  ELEMENTS 

Systematic  Relations  between  Elements.  —  We  might 
conceive  a  state  of  affairs  in  which  all  of  the  chemical  ele- 
ments would  be  wholly  different  from  one  another,  but 
everyone  knows  that  this  is  not  actually  the  case  in  na- 
ture. When  the  elements  are  compared  it  is  found  that 
curious  resemblances  exist  among  them,  and  moreover 
that  these  resemblances  depend  in  some  way  upon  the 
atomic  weights  of  the  elements  which  are  compared. 
Take  for  example  the  three  elements,  chlorine,  bromine 
and  iodine,  which  everyone  who  has  handled  them  in 
the  chemical  laboratory  knows  to  be  curiously  similar. 
We  find  upon  investigation  that  the  atomic  weight  of 
bromine  is  approximately  the  arithmetic  mean  between 
those  of  chlorine  and  iodine.  Several  other  almost  equally 
striking  " triads"  (or  groups  of  three)  of  this  character 
can  be  pointed  out. 

The  systematic  distribution  of  the  properties  of  the  ele- 
ments becomes  still  more  obvious  when  we  arrange  them 
in  order  of  their  atomic  weights,  as  hi  the  table  below. 
It  then  appears  that,  with  certain  exceptions,  every 
eighth  element  in  the  series  possesses  similar  properties. 
This  fact  is  indicated  in  the  table  by  placing  the  names 
of  the  elements  which  resemble  each  other  in  the  same 

[68] 


Sec.  6]  FORM    OF  THE  TABLE 

vertical  columns.  Such  elements  are  said  to  belong  to 
the  same  "family,"  or  " group,"  and  it  will  be  observed 
that  there  are  eight  families  of  this  sort  in  the  table.  All 
members  of  one  family  tend  to  have  the  same  combining 
power,  or  "valency"  (see  Section  34,  below),  and  their 
compounds  with  other  elements  tend  to  resemble  each 
other  closely.  Thus  hi  the  second  column,  lithium,  so- 
dium, potassium,  rubidium  and  caesium  are  all  metals 
forming  strongly  alkaline  compounds  with  hydrogen  and 
oxygen,  of  which  ordinary  "washing  soda"  is  an  ex- 
ample. Each  of  these  elements  has  normally  a  combin- 
ing power  of  one,  that  is  one  atom  of  each  of  them  will 
unite  with  one  atom  of  such  an  element  as  chlorine,  which 
in  turn  unites  with  one  atom  of  hydrogen.  As  we  pass 
from  left  to  right  in  the  table  the  combining  power  of  the 
represented  elements  increases  and  then  diminishes, 
periodically.  Copper,  silver  and  gold  which  are  classed 
in  the  second  family  differ  from  the  other  five  elements 
in  the  family  but  form  a  transition  group  to  those  in  the 
third  column. 

Nearly  all  of  the  properties  of  the  elements  are  found 
to  depend  upon  their  position  in  the  "periodic  table," 
as  the  arrangement  which  we  are  discussing  is  called. 
This  is  true  not  only  of  the  chemical  properties  but  also 
of  such  physical  properties  as  melting  point,  specific 
gravity,  etc. 

The  elements  of  a  single  horizontal  line  are  said  to  be 
members  of  the  same  "series"  or  "period."  The  table 
is  called  "periodic,"  for  the  reason  that  as  we  pass  from 
one  member  of  a  "series"  to  the  next,  the  properties  of 
the  elements  first  depart  from  those  of  the  element  with 
which  the  series  started  and  then  alter  their  course  of 
variation  to  return  to  properties  closely  resembling  those 
of  the  first  member  of  the  series. 

[69] 


THE  PERIODIC   TABLE  [Sec.  6 

Defects  in  the  Table.  —  It  will  be  noted  that  certain 
blank  spaces  occur  in  the  table  as  we  have  represented  it. 
These  spaces  are  left  vacant  for  occupancy  by  elements  as  yet 
undiscovered.  When  the  table  was  first  constructed  by 
Mendelejeff  there  were  more  empty  spaces  in  it  than 
there  are  now.  The  faith  of  chemists  in  the  table  was  so 
great  that  they  were  led  to  predict  the  discovery  of  ele- 
ments having  atomic  weights  permitting  them  to  fit  into 
these  vacant  spaces,  and  they  even  went  so  far  as  to 
specify  the  properties  of  these  elements  and  of  then* 
compounds.  In  several  cases  these  predictions  have 
since  been  fulfilled  by  the  actual  discovery  of  the  ele- 
ments in  question. 

As  can  be  seen  by  inspection  the  periodic  table  is  not  a 
perfect  system  for  the  classification  of  the  elements,  or 
if  so  is  far  from  simple  in  many  respects.  All  of  the  ele- 
ments do  not  fit  nicely  into  their  places,  this  being  es- 
pecially true  in  the  case  of  the  nine  which  are  represented 
in  the  ninth  column.  Here  we  have  groups  of  closely  sim- 
ilar elements  which  are  of  about  the  same  atomic  weight 
and  which  refuse  to  fit  into  the  scheme  of  "  families." 

Significance  of  the  Periodic  Relationships.  —  The  exact 
meaning  of  the  definite  relationships  between  the  ele- 
ments which  are  proven  by  the  periodic  table  to  exist, 
is  at  present  largely  a  mystery.  However,  there  can  be 
little  doubt  that  the  demonstrated  resemblances  depend 
in  some  way  upon  the  constitution  of  the  atoms  of  which 
the  elements  are  made  up.  (See  Section  53,  below.) 
The  fact  that  the  properties  of  the  elements  appear  to  be 
determined  (at  least  approximately)  by  the  relative 
weights  of  their  atoms  clearly  suggests  this  interpreta- 
tion, for  if  the  atoms  have  a  definite  internal  structure  and 
if  the  units  of  this  structure  are  similar  for  all  of  the  atoms, 
increasing  complexity  would  necessarily  mean  increasing 

[70] 


Sec.  6]  THEORY   OF  THE  TABLE 

atomic  weight.  If  the  units  of  structure  were  not  similar 
we  should  hardly  expect  to  find  the  systematic  resem- 
blances which  actually  exist. 

One  of  the  earliest  theories  to  make  use  of  the  idea  of 
a  single  substance  or  "protyle"  common  to  all  of  the 
chemical  elements  was  that  of  Prout,  who  suggested  in 
1815,  that  the  heavier  atoms  might  be  clusters  or  con- 
densations of  various  numbers  of  hydrogen  atoms.  This 
hypothesis  received  support  from  the  fact  that  a  large 
number  of  atomic  weights  —  more  than  would  be  ex- 
pected as  a  result  of  chance  —  are  approximately  integral 
multiples  of  the  weight  of  the  hydrogen  atom.  However, 
accurate  determinations  of  the  weights  which  fail  to 
approximate  such  values  did  not  substantiate  Prout's 
original  belief  that  the  deviations  were  due  to  experi- 
mental errors.  Consequently,  in  its  primitive  form,  at 
least,  the  theory  had  to  be  rejected. 

The  new  facts  brought  to  light  in  the  study  of  radio- 
activity (see  Section  46,  below),  however,  make  it  prac- 
tically certain  that  if  hydrogen  is  not  one  of  the  funda- 
mental bricks  of  which  the  heavier  atoms  are  built,  just 
such  a  role  is  played  by  the  element  helium.  In  radio- 
activity, atoms  break  down  into  others  of  less  weight,  and 
this  loss  of  weight  always  occurs  in  single  decrements  of 
four  units.  This  is  the  mass  of  the  helium  atom,  and  it 
has  been  proved  that  every  change  in  mass  of  a  radio- 
active substance  is  accompanied  by  the  generation  of 
helium.  Besides  this,  it  has  been  pointed  out  that  a 
difference  of  four  units  between  neighboring  members  in 
the  periodic  table  is  a  very  common  one. 

The  Nucleus  Theory  of  the  Atom,  and  Isotopes.  —  How- 
ever, this  last-mentioned  rule  is  far  from  being  strictly 
obeyed,  and  it  is  certain  that  hydrogen  itself  cannot  be 
made  up  of  helium.  At  the  present  time  there  is  current 

[71] 


THE  PERIODIC  TABLE 


[Sec.  6 


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[72] 


Sec.  6] 


THE   PERIODIC   TABLE 


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Tellurium 
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[73] 


THE  PERIODIC   TABLE  [Sec.  6 

a  definite  and  well-grounded  conception  of  the  structure 
of  the  hydrogen  atom  (Cf.  Section  53),  in  accordance 
with  which  the  latter  is  composed  of  a  single  minute 
particle  of  negative  electricity  —  an  electron  —  and  a  very 
much  smaller  particle  of  positive  electricity  about  which 
the  electron  revolves.  Practically  the  entire  mass  of  the 
atom  is  concentrated  in  the  positive  "nucleus,"  as  it  is 
called.  The  more  ponderous  atoms  are  supposed  to  be 
formed  from  a  larger  number  of  positive  and  negative 
particles,  according  to  their  respective  weights.  The 
positive  particles  are  always  condensed  into  very  small 
nuclei,  together  with  a  portion  of  the  electrons,  and  it  is 
probable  that,  hi  the  case  of  the  heavier  elements  at  least, 
the  helium  atom  is  an  important  secondary  unit  in  the 
structure  of  these  nuclei. 

Within  the  past  few  years  the  study  of  radio-active  sub- 
stances has  brought  out  facts  which  indicate  that  the 
fundamental  principle  of  the  periodic  table  —  that  the 
chemical  properties  of  an  element  are  determined  by  its 
atomic  weight  —  is  only  approximately  true.  As  already 
mentioned  (Section  5,  above),  radio-active  elements  — 
isotopes  —  are  known  which  differ  in  atomic  weight  but 
have  identical  chemical  properties.  On  the  other  hand, 
there  are  elements  of  the  same  atomic  weight  which  show 
radical  differences  chemically.  The  explanation  of  this 
very  important  discovery  apparently  lies  in  the  sugges- 
tion that  the  chemical  nature  of  an  atom  depends  not 
directly  upon  its  weight  but  upon  its  electrical  structure, 
which  to  a  limited  extent  may  change  practically  inde- 
pendently of  the  weight.  Of  this  more  will  be  said  in  the 
appropriate  context.  (See  Section  53.) 

Chemical  Elements  as  Atomic  Mixtures.  —  The  exist- 
ence of  " isotopes"  among  the  radio-elements  suggests 
their  presence  in  other  parts  of  the  periodic  table.  In 

[74] 


Sec.  6]  ISOTOPES 

the  latter  case  there  appears  a  possible  solution  of  the 
difficulty  that  the  majority  of  the  atomic  weights  are  not 
accurate  multiples  of  that  of  hydrogen.  It  is  conceivable, 
even  probable,  that  the  chemical  elements  —  which  we 
have  previously  regarded  as  individual  species  of  matter 
—  are  in  reality  only  types,  or  classes,  of  such  individuals, 
all  of  the  members  of  a  single  class  being  indistinguish- 
able and  non-separable  from  each  other  by  purely  chemi- 
cal means.  In  this  case  the  atomic  weights  given  in 
Table  I  (Section  5)  must  represent  averages  of  the  rela- 
tive weights  of  a  number  of  chemically  similar  atoms 
with  different  masses,  which  may  be  present  in  standard 
chemical  preparations  in  nearly  constant  but  in  unequal 
amounts.  If  this  is  true  it  is  not  surprising  that  our  em- 
pirically determined  atomic  weights  do  not  have  integral 
values. 

The  above  speculations  receive  support  from  certain 
experiments  by  J.  J.  Thomson,  not  depending  in  any  way 
upon  radio-active  phenomena,  which  suggested  that  the 
inert  atmospheric  gas,  neon,  is  really  made  up  of  two 
isotopic  constituents,  one  of  atomic  weight  about  20  and 
the  other  (meta-neon)  about  22.  F.  W.  Aston,  following 
out  this  clue,  found  that  by  the  use  of  physical  methods 
relying  on  the  relative  rates  of  diffusion  of  the  two 
components,  pure  atmospheric  neon  could  actually  be 
separated  into  two  gases  of  different  density.  Careful 
comparisons  of  samples  of  lead  derived  from  geologically 
different  sources  indicate  that  this  element,  also,  may 
be  a  mixture  of  isotopes,  an  idea  which  is  strongly  sug- 
gested by  its  relation  with  the  radio-active  substances. 

All  of  these  results  should  be  accepted  with  reserva- 
tions on  account  of  their  novelty,  but  it  must  be  admitted 
that  they  open  a  vista  of  new  insights  into  the  meaning  of 
the  periodic  table. 

[76] 


MOLECULAR   STRUCTURE  [Sec.  7 


REFERENCES 

For  a  detailed  discussion  of  the  periodic  table  see  Harry  C. 
Jones,  "The  Elements  of  Physical  Chemistry"  (1902),  pp.  18-37. 

A  more  popular  exposition  can  be  found  in  R.  K.  Duncan's  "The 
New  Knowledge  "  (1905),  Part  H. 

See  also  W.  Nernst's:  "Theoretical  Chemistry,"  Eng.  trans. 
from  6th  German  ed.  (1911),  pp.  178-190. 

On  "isotopes,"  see  Frederick  Soddy's  "The  Chemistry  of  the 
Radio-Elements"  (1915),  Part  I,  pp.  50-56;  Part  II,  complete. 
Also  an  article  by  E.  Rutherford:  "The  Constitution  of  Matter," 
Popular  Science  Monthly,  August,  1915. 


Section  7 

THE  ARRANGEMENT  OF  THE  ATOMS  IN  THE 
MOLECULE 

Atoms  may  group  themselves  to  form  molecules  of 
almost  any  conceivable  shape.  The  properties  of  the 
substance  which  such  molecules  compose  seem  to  depend 
in  large  part  upon  the  manner  in  which  the  atoms  are 
combined)  that  is,  not  only  upon  the  number  and  nature 
of  the  atoms  but  upon  their  geometrical  arrangement 
within  the  molecule. 

To  show  what  a  great  variety  of  substances  can  be 
formed  by  different  modes  of  combination  of  the  same 
elements  it  may  be  stated  that  about  two  hundred  thou- 
sand distinct  compounds  of  the  element  carbon  are  now 
known,  most  of  which  are  with  only  three  other  elements : 
hydrogen,  oxygen,  and  nitrogen.  These  substances  be- 
long to  the  class  of  "organic"  compounds,  so  called  for 
the  reason  that  many  of  them  are  essential  in  the 
chemical  structures  and  changes  of  living  organisms. 
There  are  undoubtedly  thousands,  if  not  millions,  of 
specific  chemical  substances  in  living  bodies  which  are 

[76] 


Sec.  7]  STRUCTURAL   FORMULAE 

built  up  from  the  same  four  elements  but  which  have 
not  yet  been  separated  out  and  analyzed. 

The  element  carbon  is  remarkable  on  account  of  the 
very  large  number  of  compounds  which  it  can  form,  but 
the  other  elements  also  enter  into  the  composition  of  a 
great  variety  of  different  substances.  Each  of  these 
substances,  organic  or  inorganic,  possesses  characteris- 
tic properties  which  distinguish  it  more  or  less  sharply 
from  all  other  substances. 

homers  and  Structural  Formulae.  —  That  the  differ- 
ences which  exist  in  the  properties  of  substances  must 
depend  at  least  in  part  upon  the  arrangement  of  the  atoms 
within  the  molecule  is  proved  by  the  fact  that  compounds 
exist  which  have  quite  different  properties  but  exactly 
similar  numbers  and  kinds  of  constituent  atoms.  Such 
cases  are  quite  common  in  organic  chemistry,  and  the 
substances  involved  are  known  as  "isomers."  Thus 
the  organic  chemist  is  acquainted  with  twenty-six  different 
compounds  which  contain  four  atoms  of  carbon,  six  of 
hydrogen  and  three  of  oxygen,  and  with  one  hundred  and 
fifty-seven  which  are  composed  of  ten  atoms  of  carbon 
and  sixteen  of  hydrogen. 

It  is  customary  for  the  chemist  to  represent  the  make- 
up of  a  compound  by  means  of  a  so-called  "chemical  for- 
mula" which  shows  in  a  simple  way  the  constitution  of  the 
molecules  of  the  substance.  The  formula  of  water,  H2O, 
may  be  taken  as  a  simple  example.  This  formula  states 
that  a  molecule  of  water  is  made  up  of  two  atoms  of  hydro- 
gen, H,  combined  with  one  atom  of  oxygen,  O.  As  an 
example  of  a  more  complex  formula  we  may  consider 
that  of  alcohol,  C2  H6O,  or  that  of  the  coal-tar  oil,  benzene, 
C6H6,  in  both  of  which  C  stands  for  the  element,  or  the 
atoms  of  carbon. 

There  is  only  one  substance  which  has  the  formula 
[77] 


MOLECULAR  STRUCTURE      [Sec.  7 

H2O,  so  that  no  confusion  can  arise  as  to  the  meaning 
of  this  formula.  It  always  stands  for  water.  However, 
at  least  two  substances  are  known  which  have  the 
formula  C2H6O,  namely  ordinary  alcohol,  and  a  gas  called 
* 'methyl  ether,"  which  is  closely  similar  to  the  ether 
employed  in  surgical  operations.  It  is  obvious  that  if  we 
desired  we  could  write  the  formula  of  water  as  H-O-H, 
in  order  to  show  the  probable  manner  in  which  the  atoms 
are  combined  in  the  molecule,  but  although  this  is  not 
required  in  the  case  of  water  it  is  found  necessary  hi  the 
case  of  alcohol,  and  other  substances  which  have  isomers. 
Such  a  formula  is  called  "structural"  or  "graphical" 
because  it  is  a  simple  drawing  representing  the  supposed 
arrangement  of  the  atoms  in  the  molecule  of  a  substance. 
These  formulae  can  be  constructed  by  studying  the  chemi- 
cal relationships  which  exist  between  different  com- 
pounds, and  when  thus  evolved  they  not  only  enable  us 
to  distinguish  theoretically  between  isomers,  but  also 
explain  the  differences  which  are  found  hi  their  properties. 
However,  as  we  shall  see,  their  utility  is  not  limited  to 
the  study  of  isomerism,  for  it  is  obvious  that  the  more 
exact  is  our  knowledge  of  the  structure  of  different  mole- 
cules the  clearer  will  be  our  ideas  concerning  the  changes 
which  are  liable  to  occur  in  these  molecules. 

A  study  of  the  manner  hi  which  alcohol  can  be  built 
up  from  its  elements  leads  us  to  assign  to  it  the  structural 

HH 

I    I 

formula:  H-C-C-O-H.    The  substance  "methyl  ether," 
I    i 
HH 

on  the  other  hand,  which  we  have  mentioned  as  an  isomer 

H       H 

I          | 

of  alcohol,  proves  to  have  the  formula:     H-C-O-C-H. 

[  78  ]  H       H 


Sec.  7]  ISOMERIC   COMPOUNDS 

H    H    H    H    H    H 
Normal  Hexane  H—  C—  C—  C—  C—  C—  C— H 


H  H 

H—  C C— H 

H  | 

I  H 

Methyldiethyl  Methane  H—  C C— H 

H 
H  I 

H—  C C— H 

I  I 

H  H 


H 

H— C— H     H     H     H 

Dimethylpropyl  Methane  H—  C —    —  C—  C—  C— H 

H— C— H     H     H     H 

H 


H  H 

H—  C— H  H—  C— H 

I  | 

Dimethylisopropyl  Methane                 H—  C-  —  C— H 

H—  C— H  H—  C— H 


H 

H— C— H 
H  H 

Trimethylethyl  Methane  H— C—    —  C—    -  C— H 

H 

H— C— H 

H— C— H 

TT 

Fig.  12 

FIVE  ISOMERIC  HYDROCARBONS  HAVING   THE 
CONSTITUTION  C6H14 

These  chemical  formulas  represent  the  supposed  structure  of  the  mole- 
cules of  five  distinct  substances  all  of  which  contain  the  same  number  of 
hydrogen  and  of  carbon  atoms. 

[79] 


MOLECULAR   STRUCTURE  [Sec.  7 

It  is  easy  to  see  why  the  decomposition  of  the  two  mole- 
cules thus  represented  should  lead  to  different  results  in 
spite  of  the  fact  that  identically  the  same  atoms  are 
present  in  each  case.  We  believe  that  these  formulae 
give  a  partial  representation  of  the  actual  arrangement 
of  the  atoms  in  the  molecules  of  alcohol  and  methyl  ether 
respectively. 

Figure  12  gives  the  graphical  formulae  of  five  isomeric 
hydrocarbons  each  made  up  of  six  atoms  of  carbon, 
and  fourteen  of  hydrogen.  Although  the  formulae  show 
certain  general  resemblances,  the  five  structures  are 
nevertheless  quite  distinct.  The  resemblances  corre- 
spond with  an  actual  physical  similarity  of  the  sub- 
stances, which  causes  them  to  be  grouped  in  the  same 
general  class,  but  within  this  class  the  substances  show 
a  perfectly  clear  chemical  individuality.  Many  other 
examples  of  this  principle  that  the  nature  of  a  chemical 
substance  depends  upon  the  exact  geometric  structure 
of  its  molecules  could  easily  be  found. 

The  "Benzene  Ring"  -  The  approximate  truth  of  the 
representations  of  the  structure  of  molecules  which  are 
given  by  the  structural  formulae  now  in  use  among  chem- 
ists, is  attested  by  such  cases  as  that  of  the  formula  of 
benzene,  the  coal-tar  oil  of  which  we  have  spoken  above. 
The  six  carbon  atoms  are  supposed  to  be  arranged  in  a 
molecule  of  this  substance  in  the  form  of  a  ring,  and  to 
each  of  them  is  attached  one  of  the  hydrogen  atoms. 
This  formula  is  shown  in  Figure  13,  (a).  Each  of  the 
hydrogen  atoms  in  the  molecule  can  be  replaced  by  atoms 
of  other  elements,  as  for  example  chlorine  atoms,  and  for 
every  new  and  different  molecule  thus  produced  there 
should  exist  a  correspondingly  distinct  substance,  which 

[80] 


Sec.  7]  BENZENE   DERIVATIVES 


H 


H—  C     C—  H  H—  C     C—  Cl 

H-C     C-H  H-C     C-H 
V          V 


C.  H 

C  t 

X  \  /  \         /  \ 

Cl— C     C— H  Cl— C     C— Cl  Cl— C     C— H 

H— C     C— H  H— C     C— H  H— C     C— Cl 

V  V    V 

i  Jl 


H 

t    t    t 

X  \          /  \         /  \ 

Cl—  C     C—  Cl  Cl—  C     C—  Cl  Cl—  C     C—  Cl 

H—  C     C—  Cl  H—  C     C—  H  H—  C     C—  H 

V     V     V 


i 


Fig.  13  a 
See  Fig.  13  b 

[81] 


MOLECULAR   STRUCTURE  [Sec.  7 


Cl 


I: 


/      \  /      V  /  \ 

H—  C  C—  Cl  H—  C  C—  H  H—  C  C—  Cl 

H—  C  C—  Cl  Cl—  C  C—  Cl  Cl—  C  C—  H 

V  V 


d, 


t 


C, 

t 

/  \  /  \ 

H—  C  C—  Cl    Cl—  C  C—  Cl 

II  I  II  I 

Cl—  C  C—  Cl    Cl—  C  C—  Cl 

\  X  V  X 

C  C 


A 


Fig.  13  b 

BENZENE  AND  ITS   CHLORINE  DERIVATIVES 
The  significance  of  these  formulae  is  explained  in  the  text. 

would  be  known  to  the  chemist  as  a  "chlorine  derivative 
of  benzene." 

Now  a  moment's  study  will  show  that  by  a  simple  exami- 
nation of  the  ring  formula  we  can  predict  the  number  of 
such  derivatives  which  we  shall  be  able  to  form,  provided 
the  formula  is  correct.  If  only  one  hydrogen  atom  is 
replaced  there  is  only  one  possible  compound,  (b)  Figure 
13,  since  the  ring  is  perfectly  symmetrical  and  hence  the 
structure  which  is  formed  is  the  same  no  matter  which 
hydrogen  atom  is  disturbed.  However,  if  two,  three,  or 

[82] 


Sec.  7]  MOLECULES  OF  ELEMENTS 

four  hydrogen  atoms  are  replaced  there  will  be  three  dif- 
ferent molecular  structures  corresponding  to  each  of  these 
numbers.  This  fact  is  shown  in  Figure  13  (c)  to  (k) 
inclusive.  No  more  than  three  can  be  formed,  however, 
in  each  case.  If  five  or  six  chlorine  atoms  are  introduced 
only  one  compound  can  be  produced  corresponding  to 
each  number,  (1)  and  (m),  respectively,  in  the  Figure. 
We  are  thus  able  to  prophesy  the  possibility  of  twelve 
chlorine  derivatives  of  benzene  and  of  no  more  than 
twelve.  The  actual  study  of  this  substance  in  the  labo- 
ratory has  revealed  the  existence  of  all  of  these  deriva- 
tives and  has  proven  the  impossibility  of  producing  any 
others.  This  is  a  striking  verification  of  the  idea  that  the 
benzene  molecule  actually  has  a  ring  structure. 

We  have  studied  this  matter  of  the  structure  of  the 
molecule  in  connection  with  benzene  because  the  formula 
of  this  substance  is  distinctive  and  is  one  of  the  most 
successful  in  its  applications.  However,  there  are  other 
instances  of  the  same  thing  which  are  almost  equally 
striking.  Indeed  the  science  of  organic  chemistry  would 
be  practically  impossible  without  the  help  which  is  pro- 
vided by  a  knowledge  of  the  actual  structure  of  the  mole- 
cules composing  the  substances  with  which  it  deals.  The 
exact  structure  of  the  molecule  is  of  less  importance  hi 
inorganic  chemistry  because  here  the  molecules  are  so 
much  simpler. 

Molecules  of  Single  Elements.  —  In  this  connection  it 
may  be  well  to  note  the  fact  that  the  atoms  of  the  elements 
in  the  pure  state  generally  unite  to  form  molecules,  which 
are  thus  made  up  of  two  or  more  atoms  of  the  same  kind. 
Thus  hydrogen  gas  is  not  composed  of  free  atoms  but  of 
hydrogen  molecules,  each  of  which  contains  two  atoms  of 
the  element.  Many  simple  elements  in  gaseous  form 
have  two  atoms  hi  then*  molecules.  The  vapor  of  the 

[83] 


MOLECULAR  STRUCTURE 


[Sec.  7 


metal  mercury  is  distinguished  from  most  elementary 
gases  by  the  fact  that  its  atoms  are  uncombined.  Some 
elements,  on  the  other  hand,  form  molecules  containing  as 
many  as  seven  similar  atoms,  and  the  same  element  may 
yield  molecules  of  different  sizes  under  different  condi- 


// 


Fig.  14 

MODELS  OF  TARTARIC  ACID  MOLECULES 

To  gain  a  correct  impression  from  this  drawing  one  should  imagine  the 
H  and  OH  circles  on  the  inner  portion  of  each  model  to  be  spheres  pro- 
jecting outward  from  the  page,  so  that  the  models  have  a  three-dimen- 
sional form.  The  letters  attached  to  each  black  circle  stand  for  the  groups 
of  atoms  which  the  circle  represents,  and  the  lines  connecting  the  circles 
indicate  the  structure  of  the  molecules.  It  will  be  observed  that  these 
two  molecules,  which  are  of  "right "and  " lef t "  tartaric  acid  respectively, 
are  so  constructed  that  one  is  the  mirror-image  of  the  other.  This  rela- 
tionship of  structure  is  offered  as  an  explanation  of  the  similar  relationship 
which  exists  between  the  structure  of  the  crystals  shown  in  Figure  15. 

tions.  The  various  forms  of  pure  sulphur  and  of  phos- 
phorus— the  yellow  and  the  red — probably  correspond 
to  molecules  containing  different  numbers  of  atoms  of 
these  elements.  It  is  possible  that  the  three  familiar 

[84] 


Sec.  7] 


MOLECULES  AND  CRYSTALS 


forms  of  the  element  carbon:   charcoal,  graphite,  and 
diamond,  may  be  due  to  the  same  causes. 

Molecular  and  Crystal  Structure.  —  The  structure  of  the 
molecule  which  is  characteristic  of  a  substance  is  proba- 
bly closely  related  with  the  shape  of  the  crystals  which 
it  produces.1  Nearly  all  pure  substances  will  take  on  a 
characteristic  crystalline  shape  under  the  right  condi- 
tions. There  are  certain  pairs  of  sugars  —  compounds  of 


CRYSTALS   OF 


Fig.  15 
1  RIGHT"   AND    "LEFT1 


TARTARIC   ACIDS 


It  will  be  observed  that  the  two  crystals  represented  above  are  identical 
inform  except  for  the  fact  that  one  is  the  mirror-image  of  the  other;  what 
is  on  the  right-hand  side  of  one  is  on  the  left-hand  side  of  the  other.  The 
two  crystalline  forms  represent  two  different  kinds  of  tartaric  acid,  but 
ordinary  chemical  analysis  reveals  no  difference  in  their  composition. 
It  is  supposed  that  the  actual  basis  of  the  distinction  lies  in  the  fact  that 
the  molecules  of  the  two  acids  differ  in  the  same  general  way  in  which 
the  crystals  differ.  These  molecules  are  symbolized  in  Figure  14. 

carbon,  hydrogen,  and  oxygen  —  the  molecules  of  which 
as  represented  in  their  structural  formulae,  are  distin- 
guished from  each  other  only  by  the  fact  that  one  is  the 
mirror-image  of  the  other.  This  difference  is  made  clear 
in  the  accompanying  Figure  14.  Now  it  turns  out  that 
when  these  sugars  crystallize,  although  the  crystals  do 
not  have  the  same  form  as  the  molecules,  they  do  differ 

1  Present-day  studies  of  crystal  constitution  (see  Section  22) 
show  that  the  atom  and  not  primarily  the  molecule  is  the  unit  of 
structure. 

[85] 


PHYSICAL  PROPERTIES  [Sec.  8 

in  the  same  way  in  which  the  molecules  differ,  i.e.,  one  is 
the  mirror-image  of  the  other.  This  fact,  which  is  shown 
by  a  comparison  of  Figures  14  and  15,  seems  to  prove 
quite  conclusively  that  the  shape  of  the  crystal  depends 
directly  upon  that  of  the  molecule. 

In  this  connection  it  is  interesting  to  note  that  whereas 
one  of  these  sugars  —  either  in  the  crystalline  form  or  in 
solution  —  turns  polarized  light  to  the  left,  the  other 
turns  it  to  the  right,  and  in  exactly  the  same  proportion. 
This  fact  speaks  for  the  truth  of  the  formulae  which  have 
been  assigned  to  the  compounds. 

Further  considerations  with  regard  to  crystal  structure 
will  be  found  in  Section  22. 

REFERENCES 

An  excellent,  detailed  and  not  very  difficult  discussion  of  "The 
Constitution  of  the  Molecule,"  will  be  found  in  W.  Nernst's 
"  Theoretical  Chemistry,"  English  translation  from  6th  German 
edition  (1911),  Book  H,  Chapter  4,  pp.  278-300. 

Simpler  considerations  appear  in  F.  J.  Moore's  "  Outline  of 
Organic  Chemistry"  (1910).  Chapter  VIII,  pp.  147-161,  deals 
with  the  phenomena  of  crystal  form  above  mentioned. 

Section  8 

THE   PHYSICAL  PROPERTIES   OF  COMPOUND 
SUBSTANCES 

Importance  of  the  Internal  Molecular  Forces.  —  We 
have  asserted  in  Section  7,  above,  that  the  properties  of 
compound  substances  depend  principally  upon  the  man- 
ner in  which  the  atoms  are  arranged  in  the  molecule. 
Strictly  speaking,  however,  we  must  say  that  the  char- 
acteristic properties  of  a  substance  depend  upon  the 
strength  and  arrangement  of  the  forces  of  attraction  which 
hold  the  molecule  together.  We  perceive  such  qualities 
of  bodies  as  hardness,  elasticity,  color,  odor,  etc.,  only 

[86] 


Sec.  8]  COLOR 

because  these  bodies  bring  to  bear  upon  our  organs  of 
touch,  sight,  and  smell  certain  characteristic  combina- 
tions of  forces.  The  nature  of  these  forces  must  always 
depend  at  least  in  part  upon  the  nature  of  the  forces  within 
and  between  the  molecules  of  which  the  body  is  made  up. 

Hard  bodies  are  those  in  which  the  forces  which  exist 
between  the  molecules  are  very  strong  and  hold  them 
closely  together,  so  that  the  body  cannot  be  distorted  by 
our  touch.  An  elastic  body  is  one  in  which  the  same 
forces  act  to  restore  its  original  shape,  once  it  has  been 
distorted.  Among  other  characteristic  properties  of  com- 
pound substances  which  must  be  determined  by  the 
internal  and  external  forces  of  the  molecule  may  be 
mentioned  their  melting  and  freezing  points,  their  latent 
heats  of  vaporization  and  of  fusion,  their  dielectric  con- 
stants (or  the  degree  to  which  they  alter  the  intensity  of 
an  electrical  field  in  which  they  are  placed),  their  optical 
nature,  their  surface  tension  and  the  pressure  exerted  by 
their  vapor  when  in  the  liquid  state,  their  chemical  activ- 
ity, the  forms  of  then*  crystals,  then*  compressibility, 
viscosity,  magnetic  power,  etc.  In  the  course  of  the  dis- 
cussion in  both  Part  I  and  II,  the  manner  in  which  these 
properties  are  determined  by  molecular  and  inter-molec- 
ular forces  will  gradually  be  made  clear. 

Color.  —  The  colors  of  substances  depend  upon  the 
nature  of  the  light  which  they  absorb  and  reflect.  If  a 
body  looks  red  in  white  light  this  means  that  it  absorbs 
a  great  deal  of  green  and  blue  light  and  reflects  a  rela- 
tively large  amount  of  red.  This  power  to  absorb  one 
light  and  to  reflect  another  is  known  to  depend  directly 
upon  the  strength  of  the  forces  which  hold  the  electrons 
in  the  molecule,  as  is  explained  in  Section  41,  below. 

The  nature  of  the  elements  of  which  compounds  are 
made  up  is  undoubtedly  of  the  utmost  importance  in 

[87] 


PHYSICAL  PROPERTIES  [Sec.  8 

determining  their  properties,  but  it  is  probable  that  the 
elementary  atoms  are  effective  primarily  through  their 
power  to  regulate  the  forces  within  the  compound  mole- 
cules. Thus  the  blue  color  of  many  copper  compounds  is 
due  to  the  copper  atom  common  to  all.  But  since  this 
color  changes  to  red  or  brown  when  certain  well-known 
changes  occur  in  the  forces  binding  the  molecules  of  such 
compounds  together,  we  are  compelled  again  to  conclude 
that  what  may  be  called  the  "  dynamical  (or  force)  con- 
stitution of  the  molecule"  is  the  immediate  cause  of  the 
physical  properties  of  the  corresponding  substance. 

Allotropism.  —  The  marked  differences  which  exist 
between  the  so-called  "allotropic"  forms  of  certain 
elements  (such  as  carbon;  see  Section  7)  obviously  can- 
not be  attributed  to  differences  in  the  elementary  con- 
stitution of  the  substances,  and  hence  must  be  explained 
in  terms  of  the  different  arrangement,  and  degree  of  ex- 
haustion, of  the  same  atomic  forces.  Diamond  —  which 
is  one  of  the  hardest  substances  known  —  is  made  up  of 
exactly  the  same  element  as  charcoal  and  graphite,  which 
are  relatively  soft. 

The  Mystery  of  Chemical  Change.  —  Accordingly,  re- 
flection should  free  us  of  the  mystery  which  usually 
attaches  to  the  qualitative  modifications  of  the  properties 
of  bodies  which  occur  in  chemical  changes.  Molecules 
are  not  merely  chance  "heaps"  of  different  atoms. 
They  are  definite  and  relatively  stable  individuals,  the 
natures  of  which  depend,  of  course,  upon  the  forces  latent 
in  the  atoms  which  make  them  up,  but  which  neverthe- 
less realize  in  their  own  constitution  " force  patterns" 
which  do  not  exist  elsewhere.  Each  new  atomic  com- 
pound means  a  new  system  of  such  forces,  and  conse- 
quently a  new  substance,  possessing  an  individuality  of 
its  own. 

[88] 


Sec.  8]  CHEMICAL   CHANGE 

Recent  developments  connected  with  the  study  of  iso- 
topism  (see  Section  6),  make  it  very  probable  that  the 
principal  physical  and  chemical  properties  of  elementary 
substances  depend  directly  only  upon  the  number  and 
arrangement  of  the  electrons  in  the  outer  shell  of  an 
atom.  This  superficial  structure  is  identical  in  isotopes, 
although  the  inner,  nuclear  formations  differ.  Now,  there 
is  little  doubt  that  it  is  the  outer  or  " valency"  electrons 
(Cf.  Section  34),  which  are  active,  and  change  their  posi- 
tions, in  chemical  reactions.  Consequently,  it  is  natural, 
if  what  has  just  been  said  is  true,  that  the  properties  of 
compounds  should  differ  radically  from  those  of  the  ele- 
ments which  go  to  make  them  up.  Only  a  relatively 
small  portion  of  a  complex  atom  is  involved  in  its  every- 
day dealings  with  the  external  world.  The  immediately 
interesting  things  about  an  atom,  so  to  say,  are  all  super- 
ficial, and  are  easily  modified  through  intercourse  with 
other  atoms.  From  the  point  of  view  of  chemistry,  it  is 
possible  that  the  atom  may  be  more  radically  altered  in 
a  chemical  reaction  than  in  a  radio-active  transformation, 
although  the  latter  is  fundamental  and  irrevocable,  and 
the  former  easily  reversible. 

REFERENCES 

The  details  involved  in  the  above  discussion  will  be  further  con- 
sidered in  subsequent  sections  in  which  references  will  be  given 
to  the  special  topics  concerned. 

For  a  more  detailed  general  discussion  see  W.  Nernst's  "Theo- 
retical Chemistry"  (1911),  Book  H,  Chapter  5,  pp.  303-347,.  and 
Norman  Campbell's  "Modern  Electrical  Theory,"  second  edition 
(1913),  Chapter  XII. 

A  good  discussion  of  color  appears  in  Franklin  and  MacNutt's 
"Light  and  Sound"  (1909),  Chapter  X.  See  also  M.  Luckiesh's 
excellent  volume,  "Color  and  its  Applications"  (1915). 


[89] 


CHEMICAL  EQUATIONS  [Sec.  9 

Section  9 
CONCERNING  CHEMICAL  EQUATIONS 

The   chemist   is   accustomed   to   represent   chemical 
changes  by  means  of  equations,  such  as  the  following : 

H20  =  2  H  +  O, 

which  is  intended  to  show  how  water  breaks  down  into 
hydrogen  and  oxygen.  The  symbols  on  the  left-hand  side 
of  the  equation  represent  the  substances  entering  into 
the  reaction,  and  those  on  the  right-hand  side  represent 
its  products.  The  equation  above,  stands  for  a  chemical 
change  of  a  purely  destructive  type.  If  the  direction  of 
the  change  is  reversed,  we  have: 

2  H  +  O  =  H2O 

which  is  a  constructive  reaction.  However,  very  few  chem- 
ical changes  are  purely  destructive  or  purely  constructive. 
As  a  rule,  there  is  simultaneous  building  up  and  break- 
ing down.  Thus  the  actual  process  which  occurs  when 
water  is  decomposed  into  hydrogen  and  oxygen  is  not 
so  simple  as  we  have  represented  it  in  the  first  equation 
above,  but  is  more  accurately  symbolized  by  the  follow- 
ing relationship: 

2  H2O  =  2H2  +  O2 

This  reaction  probably  goes  on  in  two  stages,  the  first 
being  that  indicated  in  the  equation  which  was  originally 
given,  and  the  second  being  the  combination  of  the  free 
hydrogen  and  oxygen  atoms  thus  produced,  to  form  the 
hydrogen  and  oxygen  molecules,  H2  and  O2,  respectively, 
which  we  mentioned  in  Section  7.  However,  since  the 
chemist  is  usually  interested  in  the  so-called  "end  prod- 
ucts" of  a  reaction,  and  since  in  any  case  the  two  lines 

[90] 


Sec.  10]  FORCES   OF   COHESION 

of  change  are  continuous  with  each  other,  the  reaction  is 
ordinarily  written  as  shown  above. 

Section  10 
THE  FORCES  OF  ATTRACTION  WITHIN  BODIES 

Everybody  is  aware  of  the  fact  that  particles  of  matter 
attract  each  other  with  a  force  which  is  greater  the  nearer 
the  particles  are  together.  As  everyone  knows,  it  is  the 
gravitational  attraction  between  the  earth  and  the  bodies 
upon  it  which  causes  the  latter  to  have  "weight."  Now 
since  all  bodies  are  made  up  of  atoms  it  follows  that  the 
forces  of  gravity  must  depend  in  some  way  upon  attrac- 
tions which  atoms  exert  u'pon  each  other,  and  on  account 
of  the  fact  that  the  atoms  are  separated,  at  least  in  the 
case  of  solids  and  liquids,  by  very  minute  distances  we 
should  expect  these  " inter-atomic"  forces  to  be  relatively 
more  powerful  than  are  those  of  ordinary  gravitation. 
But  as  far  as  the  atoms  are  concerned  gravitation  is  only 
a  sample  of  much  more  powerful  forces,  for  the  former  is 
in  all  probability  a  mere  residue  from  the  latter. 

At  the  present  time  the  nature  of  the  relationship  which 
almost  certainly  exists  between  gravitation  and  the 
mutual  attractions  of  the  atoms  is  largely  a  mystery,  but 
strange  as  it  may  seem,  we  are  much  clearer  concerning 
the  connection  between  atomic  and  molecular  attractions, 
between  "chemical  affinity"  and  the  forces  of  cohesion 
within  bodies. 

As  we  have  suggested  in  Section  6,  the  atoms  are 
probably  complex,  being  made  up  of  ultimate  particles 
which  are  much  smaller  than  the  atoms  themselves.  On 
account  of  the  great  stability  of  atoms  we  must  suppose 
these  particles  to  be  held  in  position  by  very  powerful 
forces  of  attraction.  These  forces,  which  are  probably 

[91] 


THE  KINETIC   THEORY  [Sec.  11 

electrical,  are  not  perfectly  balanced  within  the  atoms  and 
hence  tend  to  be  effective  in  causing  them  to  adhere  to 
each  other.  Such  secondary  attraction  is  probably  the 
basis  of  what  is  commonly  called  "chemical  affinity," 
the  force  which  binds  atoms  together  into  molecules. 
But  just  as  the  tendencies  of  attraction  are  not  wholly 
exhausted  within  the  atom,  so  there  are  residual  attrac- 
tions which  remain  after  the  molecule  has  been  formed, 
and  it  is  these  secondary  residual  forces  which  underlie 
the  properties  of  cohesion,  elasticity,  and  rigidity  in 
solids  or  liquids.  Their  existence  also  accounts  for  cer- 
tain striking  characteristics  of  gases,  as  well  as  of  solids 
and  liquids,  as  we  shall  see  later  on  in  our  discussion. 

It  is  important  that  the  reader  should  bear  in  mind  the 
qualitative  identity  of  these  different  forces  of  attrac- 
tion which  operate  between  the  particles  of  which  all 
bodies  are  composed,  and  also  the  nature  of  their  quanti- 
tative relationships. 

REFERENCES 

On  the  relation  between  atomic,  chemical  and  cohesion  forces, 
see  Sir  Oliver  Lodge's  book  on  "Electrons"  (1906),  Chapter  XVI. 

Also  Norman  Campbell's  "  Modern  Electrical  Theory,"  second 
edition  (1913),  Chapters  XII  and  XIII. 

Section  11 
"THE  KINETIC   MOLECULAR  THEORY" 

The  Nature  and  Foundations  of  the  Theory.  —  The 
" proof"  of  the  doctrine  that  the  atoms  of  all  bodies  are 
in  constant  motion  has  been  given  by  the  so-called  "  ki- 
netic molecular  theory,"  hi  connection  with  the  results 
of  experiment.  This  theory  may  well  be  described  as  an 
application  of  the  laws  of  mechanics,  or  "  dynamics," 
to  the  world  of  molecules.  The  fundamental  principles 

[92] 


Sec.  11]       MOLECULES   AND   PROBABILITY 

of  mechanics  are  the  familiar  "laws  of  motion"  of  New- 
ton, and  it  is  a  very  significant  fact  that  these  principles 
which  were  first  applied  successfully  to  astronomical 
bodies  should  apply  also  to  the  almost  infinitely  smaller 
bodies  called  molecules.  Certain  very  recent  considera- 
tions tend  to  limit  the  applicability  of  these  laws  (see 
Section  54,  below),  but  they  certainly  apply  approxi- 
mately, and  on  the  average,  to  a  wide  variety  of  molecular 
happenings. 

The  kinetic  theory  regards  each  molecule  as  an  inde- 
pendent being,  endowed  with  motion  and  having  certain 
attractions  for  all  other  molecules.  It  then  proceeds  to 
investigate  the  effects  which  should  follow  from  the 
presence  of  a  very  large  number  of  such  molecules  in  a 
limited  space,  making  use  only  of  the  laws  of  motion,  of 
geometry,  and  of  arithmetic. 

Molecular  Chances,  and  Averages.  —  It  was  shown  by 
Maxwell,  who  may  be  considered  the  founder  of  the 
kinetic  theory,  that  most  of  the  laws  which  govern  such 
a  melee  of  vibrating  particles  must  be  "statistical"  in 
character,  that  is,  that  they  must  depend  upon  the 
average  of  a  large  number  of  different  individual  molecular 
effects.  Because  of  the  vast  number  of  molecules  which 
are  contained  in  even  a  small  volume  of  material  sub- 
stance, these  averages  are  very  certain.  When  a  small 
number  of  molecules  are  considered,  however,  their 
action  proves  to  be  less  certain. 

The  modern  theory  of  matter  has  thus  given  rise  to 
a  branch  of  inquiry  which  is  often  called  "statistical 
mechanics,"  because  it  applies  the  principles  of  statistical 
investigation  to  mechanical  problems.  These  principles 
are  essentially  those  of  "chance"  or  "probability." 
We  can  tell  what  will  happen  in  the  molecular  world  in 
about  the  same  way  in  which  we  can  predict  the  events 

[93] 


MOLECULAR   SPEEDS  [Sec.  12 

which  occur  in  human  society,  although  generally  with 
greater  accuracy.  It  is  possible  for  insurance  men  to 
calculate  with  sufficient  accuracy  for  successful  business 
administration,  the  number  of  people  who  will  commit 
suicide  or  arson  during  a  given  period,  or  who  will  be 
killed  in  train  wrecks  or  in  automobile  accidents.  With 
regard  to  one  person  nothing  definite  could  in  general  be 
foretold,  but  the  greater  the  number  of  individuals  con- 
sidered, the  more  precisely  are  predictions  fulfilled.  It 
is  the  same  in  the  molecular  world,  and  here  the  number 
of  individuals  involved  is  vast  almost  beyond  conception, 
so  that  statistical  prophecies  are  very  reliable.  However, 
what  may  be  called  the  "  individuality  of  molecular 
activities"  is  of  great  importance  in  modern  physics. 

The  " proof"  of  the  hypothesis  of  molecular  motion 
to  which  we  have  alluded  lies  in  the  remarkable  corre- 
spondence which  exists  between  the  results  of  the  kinetic 
theory  and  the  facts  of  nature  as  determined  by  experi- 
ment, a  correspondence  which  applies  both  to  the  statis- 
tical and  to  the  individual  behavior  of  the  molecules. 

REFERENCES 

On  the  kinetic  theory,  see  W.  P.  Boynton's  "  Application  of  the 
Kinetic  Theory  to  Gases,  Vapors,  Pure  Liquids,  and  the  Theory 
of  Solutions,"  1904. 

Also  W.  Nernst's  "Theoretical  Chemistry"  (1911),  Book  H, 
Chapter  n,  pp.  197-249.  The  latter  account  is  perhaps  the  simpler 
of  the  two,  and  also  the  more  empirical. 

Section  12 
THE  SPEEDS  OF  MOLECULAR  MOTION 

The  Molecular  Counterpart  of  "  Temperature"  —  The 
exact  temperature  of  bodies  is  not  directly  proportional 
to  the  speed  at  which  then*  molecules  move,  but  rather 
depends  upon  the  average  energy  of  molecular  motion. 

[94] 


Sec.  12]  PARTITION   OF  ENERGY 

It  is  a  familiar  fact  that  the  energy,  or  "  kinetic  energy," 
of  a  moving  body  depends  not  only  upon  the  speed  at 
which  it  is  travelling  but  also  upon  its  weight  or  "mass," 
or  —  if  we  consider  only  bodies  made  up  of  the  same  sub- 
stance—  upon  its  size.  Other  things  being  equal,  the 
larger  a  body  is  the  more  work  must  be  done  to  set  it 
in  motion  or  to  stop  it,  once  it  is  moving.  It  is  found  by 
calculation  from  the  laws  of  motion  and  by  experiment, 
that  the  energy  of  motion,  or  kinetic  energy,  of  any  body 
is  proportional  to  its  mass  and  to  the  " square"  of  its 
velocity. 

This  measure  of  energy  of  motion  applies  to  molecules 
as  well  as  to  visible  bodies,  and  so  we  must  say  that  if 
the  temperature  of  a  piece  of  matter  is  proportional  to 
the  average,  kinetic  energy  of  its  molecules  its  tempera- 
ture is  proportional  to  the  average  square  of  the  speed  of 
these  molecules,  so  that  as  their  speed  increases  the 
corresponding  temperature  increases  much  more  rapidly 
in  proportion.  From  these  considerations,  also,  it  fol- 
lows that  if  we  have  a  number  of  substances  at  the  same 
temperature  the  speeds  of  their  respective  molecules  will 
on  the  average,  be  less  the  larger  (more  massive)  these 
molecules  are.  This  must  follow  because  at  the  same 
temperature  the  average  energy  must  be  the  same  re- 
gardless of  the  weight  of  the  molecules,  and  this  can 
only  be  true  if  the  heavier  molecules  are  moving  at  lower 
speeds  than  are  the  others. 

When  a  large  number  of  atoms  or  molecules  of  differ- 
ent species  and  weights  are  mixed  together  the  average 
energy  of  each  species  will  be  the  same  as  that  of  any 
other  species.  It  seems  reasonable  that  this  should  be 
the  case,  for  if  the  average  energies  were  not  equal  in 
this  way,  the  different  substances  making  up  the  mix- 
ture would  be  at  different  temperatures,  which  is  incon- 

[95] 


MOLECULAR   SPEEDS  [Sec.  12 

sistent  with  the  well-known  fact  that  all  bodies  which 
are  in  close  contact  tend  to  come  to  the  same  tempera- 
ture. As  we  shall  see  later  (Section  21),  even  when  the 
vibrating  particles  are  so  large  as  to  be  visible  under  the 
microscope,  their  average  energy  of  motion  is  approxi- 
mately the  same  as  that  of  the  very  much  smaller 
molecules.  This  is  one  aspect  of  what  is  known  as  the 
principle  of  "the  equipariition  of  energy." 

Actual  Molecular  Speeds.  —  We  have  seen  in  our  pre- 
vious discussion  that  atoms  and  molecules  differ  widely 
in  weight,  and  since  weight  and  mass  are  proportional, 
it  follows  from  what  has  been  said  above  about  the  equal- 
ity of  molecular  energies  of  different  substances  at  the 
same  temperature,  that  the  speeds  of  different  molecules 
and  atoms  under  these  conditions  will  differ  a  great  deal. 
The  average  speed  of  a  hydrogen  gas  molecule  at  a  tem- 
perature corresponding  with  the  freezing  point  of  water 
is  about  one  and  one-eighth  miles  a  second,  that  of  the 
mercury  vapor  molecule  at  the  same  temperature  is 
about  one-tenth  of  this,  or  about  six  hundred  feet  a 
second.  The  mercury  molecule  weighs  just  one  hundred 
times  as  much  as  the  hydrogen  molecule.  For  the  same 
temperature,  molecular  speeds  vary  as  the  square-roots 
of  the  molecular  masses.  Even  the  mercury  molecule 
moves  at  the  tremendous  speed  of  four  hundred  miles 
an  hour.  Vast  as  this  may  seem,  it  is  as  nothing  com- 
pared with  the  speeds  which  are  attained  by  molecules 
and  especially  by  electrons  under  other  conditions  which 
we  are  to  consider  at  another  point  hi  our  discussion 
(see  Section  25,  below). 

The  reader  should  bear  in  mind  the  fact  that  it  is  the 
average  energy  of  motion  which  is  constant  at  a  given 
temperature.  The  speeds  of  the  individual  molecules 
differ  enormously,  but  the  variations  in  one  direction 

[96] 


Sec.  13]  MEAN   FREE   PATH 

balance  those  in  the  other  so  that  from  the  statistical 
point  of  view  which  we  have  explained  in  Section  11 
there  is  constancy. 

REFERENCES 

For  a  detailed  and  simple  account  of  the  relations  of  molecular 
speeds  in  the  kinetic  theory  see  Chapter  III  of  Part  I  of  O.  E. 
Meyer's  "  The  Kinetic  Theory  of  Gases."  A  list  of  concrete  values 
for  the  speeds  of  molecules  of  thirty-one  different  substances  is 
given  on  pages  57-58  of  the  English  translation  of  this  work  (1899). 
"Molecular  and  Atomic  Energies"  are  discussed  in  Chapter  V  of 
Part  I. 

A  still  simpler  discussion  will  be  found  in  O.  D.  Risteen's  "  Mole- 
cules and  the  Molecular  Theory  of  Matter"  (1895),  Chapter  II. 

Section  13 

AVERAGE  DISTANCE  TRAVERSED  BY  A   GAS 
MOLECULE  BETWEEN   IMPACTS 

The  "Mean  Free  Path." — Since  the  molecules  of  a  gas 
are  moving  helter-skelter  it  cannot  be  expected  that  the 
distance  passed  over  by  individual  molecules  between 
" bounces"  will  always  be  the  same.  However,  from 
what  we  have  said  hi  Section  11  about  the  " statistical" 
nature  of  the  happenings  in  the  molecular  world  it  might 
be  anticipated  that  on  the  average  this  distance  would  be 
constant  for  the  same  gas  under  the  same  conditions. 
This  turns  out  to  be  the  case,  and  the  distance  in  ques- 
tion is  known  in  the  kinetic  theory  as  the  "mean  (or 
average)  free  path"  of  the  molecule. 

The  mean  free  path  of  a  gas  molecule  under  standard 
conditions  is  a  very  important  characteristic  of  the  gas 
on  account  of  the  fact  that  upon  its  magnitude  depend 
many  of  the  obvious  properties  of  the  gas  in  question.  It 
is  clear  that,  other  things  being  equal,  this  distance  will 
be  greater  the  smaller  the  moving  molecules  and  the  fewer 

[97] 


MEAN  FREE   PATH  [Sec.  13 

there  are  of  them  in  a  given  volume.  In  other  words  the 
average  uninterrupted  motion  of  the  molecules  is  greater 
the  smaller  their  chances  of  collision.  When  the  gas  is 
compressed  the  mean  free  path  is  diminished,  so  that  it 
varies  in  a  direction  opposed  to  that  of  the  change  in 
pressure. 

Properties  Depending  on  "Mean  Free  Path."  —  Among 
other  measurable  properties  of  gases  which  depend  upon 
the  magnitude  of  the  mean  free  path  of  then-  molecules 
are  to  be  mentioned  their  natural  rates  of  " diffusion" 
(see  Section  14)  and  the  ease  with  which  they  conduct 
electricity.  Any  phenomenon  which  depends  upon  the 
rate  at  which  individual  molecules  can  move  from  one 
point  to  another  hi  the  gas  body  will  also  depend  upon  the 
size  of  the  "mean  free  path."  Electricity  is  conducted 
through  gases  by  being  carried  along  bodily  on  electrons 
or  molecules  which  move  under  the  influence  of  the 
electrical  forces  exerted  by  the  dynamo  or  battery.  The 
longer  the  mean  free  path,  that  is  the  fewer  the  obstacles 
which  oppose  the  motion  of  the  electrified  molecules,  the 
less  the  "resistance"  which  is  offered  to  the  passage  of 
the  current.  This  explains  why  it  is  that  gases  under 
low  pressure  —  such  as  exist  in  many  so-called  "  vac- 
uum-tubes"—  conduct  electricity  so  much  better  than 
do  the  same  gases  at  "  atmospheric  pressure." 

For  atmospheric  gases  under  ordinary  conditions  the 
mean  free  path  is  about  one  one-millionth  of  an  inch  in 
length.  In  vacuum  tubes  for  the  same  gases  it  may  be 
greater  than  one  ten-thousandth  of  an  inch.  These  mag- 
nitudes may  seem  very  small,  but  when  we  consider  the 
fact  that  the  average  gas  molecule  is  only  one  three- 
hundred-millionth  of  an  inch  in  diameter  we  see  that 
the  free  movements  of  the  molecules  may  be  relatively 
large.  The  mean  free  path  in  hydrogen  gas  is  greater 

[98] 


Sec.  14]  DIFFUSION 

under  the  same  conditions  than  that  in,  say,  mercury 
vapor,  for  the  reason  that  the  molecules  of  the  latter 
gas  are  larger  than  those  of  the  former.  Although  the 
length  of  the  mean  free  path  varies  inversely  as  the 
square  of  the  diameter  of  the  molecules  the  difference  is 
not  very  great,  owing  to  the  relatively  small  difference 
in  diameter  of  the  two  species  of  molecules. 

On  account  of  the  great  speed  of  molecular  motion, 
the  time  intervals  between  the  successive  impacts  of  gas 
molecules  are  very  minute  even  in  highly  rarefied  gases. 

REFERENCES 

Concerning  "Molecular  Free  Paths  and  the  Phenomena  Condi- 
tioned by  Them"  consult  Part  n  of  Meyer's  "The  Kinetic  Theory 
of  Gases"  (1899). 

Section  14 
DIFFUSION 

One  very  commonly  observed  phenomenon  which  can 
be  accounted  for  only  in  terms  of  the  motion  of  molecules 
is  that  of  diffusion.  The  nature  of  this  process  is  best 
illustrated  by  the  manner  in  which  odors  travel.  When 
a  bottle  of  some  pungent  liquid  is  uncorked  in  one  corner 
of  an  apartment  the  molecules  of  the  vapor,  which  is 
always  present  above  any  liquid,  swarm  out  of  the  bottle 
and  ultimately  spread  themselves  like  insects  to  all  parts 
of  the  room.  Certain  of  them  strike  the  sensitive  sur- 
face of  the  nostrils  and  produce  distinctive  sensations  of 
smell.  Of  course  this  motion  of  odoriferous  molecules 
is  assisted  by  the  presence  of  air  currents,  but  these 
become  effective  only  when  diffusion  is  also  possible. 

The  motion  of  the  molecules  in  diffusion  can  seldom 
resemble  a  "bee  line,"  since  no  molecule  can  be  ex- 
pected to  travel  across  the  average  room  without  en- 


DIFFUSION  [Sec.  14 

countering   countless   others   in   its   course.     Diffusion 
movements    must,    therefore,    have    " jagged"    paths, 


Fig.  16 

DIFFUSION   PATHS 

These  jagged  lines  are  supposed  to  be  the  paths  of  two  gas  molecules 
which  are  followed  for  a  short  period  of  time.  They  show  quite  clearly  what 
is  meant  by  saying  that  the  motion  of  such  molecules  is  haphazard,  but, 
although  the  paths  are  very  far  from  "bee  lines,"  they  nevertheless  rep- 
resent a  progressive  displacement  of  the  molecules  from  their  original 
positions.  It  is  in  this  manner  that  gases  diffuse.  The  paths  represented 
above  were  of  course  not  obtained  by  the  observation  of  gas  molecules, 
but  they  come  from  a  closely  related  source,  viz.,  the  measurements  of  M. 
Perrin  of  the  so-called  Brownian  movement  of  small  particles  suspended 
hi  liquids  seen  under  the  microscope.  As  explained  in  the  text  the  Brown- 
ian movement  follows  the  same  laws  as  that  of  molecules  in  a  gas. 

somewhat  like  that  of  a  flash  of  lightning  (see  Figure 
16).      On  account  of  the  tremendous  obstacles  which 

[100] 


Sec.  15]  SOUND     ;'      /;,  ; 

are  opposed  to  the  straightforward  motion  of  the  mole- 
cules, the  diffusion  of  large  quantities  of  a  gas  or  vapor 
requires  long  periods  of  time. 

Diffusion  would  not  occur  at  all  if  it  were  not  for  the 
heat  motion  of  the  molecules. 

REFERENCES 

On  diffusion  see  Meyer's  work  already  referred  to,  Chapter  HI 
of  Part  H. 

An  account  of  the  physical  phenomena  of  diffusion  will  be  found 
in  A.  L.  Kimball's  "The  Physical  Properties  of  Gases"  (1890), 
Chapter  VI. 

Section  15 

SOUND 

From  the  modern  point  of  view  sound  must  be  regarded 
as  a  molecular  phenomenon. 

Sound  is  commonly  considered  to  be  a  species  of  wave- 
motion  which  is  set  up  in  the  air,  or  other  material  sub- 
stance, by  the  vibratory  motion  of  the  object  which  is 
"  emitting  the  sound."  Suppose,  for  example,  that  a 
tuning  fork  is  struck.  The  prongs  of  the  fork  are  set 
into  rapid  vibration  and  at  each  of  their  excursions  they 
push  violently  against  the  air  molecules  which  surround 
them.  These  molecules  are  thus  thrust  away  from  the 
fork  and  communicate  their  motion  to  outlying  molecules 
which  act  in  turn  upon  a  third  layer  of  molecules  further 
still  from  the  fork.  When  the  molecules  near  the  fork 
have  conveyed  the  impulse  to  the  outlying  molecules 
they  rebound,  just  as  does  a  moving  billiard-ball 
which  encounters  a  stationary  one  under  the  right 
conditions. 

It  can  be  seen  that  an  impulse  of  this  sort  will  travel 
out  from  the  source  just  as  a  wave  of  motion  passes  along 
a  row  of  standing  dominoes,  the  first  of  which  has  been 
overturned.  The  gas  molecules,  however,  unlike  the 

[101] 


LATENT  HEATS  [Sec.  16 

dominoes,  return  to  their  original  positions  when  the  wave 
or  impulse  has  passed,  and  hence  are  ready  for  the  gen- 
eration of  a  second  wave,  which  is  occasioned  by  the 
second  excursion  of  the  prong  of  the  tuning-fork.  The 
result  is  that  a  series  of  " rarefactions  and  condensations" 
among  the  molecules  travel  out  from  the  tuning-fork  and 
when  these  impinge  upon  the  ear  they  cause  the  sensa- 
tion of  sound. 

When  the  prong  of  the  tuning-fork  sets  the  air  mole- 
cules in  motion  it  of  course  endows  them  with  some 
of  its  energy,  and  it  is  this  energy  which  is  responsible 
for  the  stimulation  of  the  ear.  The  radiation  of  heat 
energy  from  a  hot  body  is  different  from  the  radiation  of 
sound  energy  in  several  ways.  In  the  first  place  heat 
energy  may  be  lost  by  two  distinct  processes.  A  hot  body 
may  lose  heat  energy  on  account  of  the  fact  that  the 
motions  of  its  surface  molecules  set  up  similar  motions 
in  the  molecules  of  surrounding  bodies,  or  it  may  lose 
energy  by  the  generation  of  "heat  waves."  These  con- 
stitute true  "radiant  heat"  and  do  not  consist  in  the 
motion  of  molecules,  as  do  sound  waves,  but  rather  are 
closely  similar  to  light,  that  is  electromagnetic  waves. 

REFERENCES 

An  excellent  semi-popular  work  "On  Sound,"  although  now  an 
old  one,  is  John  Tyndal's  book  of  that  name  (1888).  The  first 
Chapter  is  especially  pertinent.  See  also  Franklin  &  MacNutt's 
"Light  and  Sound"  (1909). 

Section  16 
LATENT  HEATS 

It  is  a  well-known  fact  that  when  a  solid  melts,  and 
that  when  a  liquid  is  vaporized,  definite  amounts  of  heat 
energy  are  absorbed,  the  so-called  latent  heats  of  fusion 
and  of  vaporization.  The  cause  of  this  absorption  of  en- 

[102] 


Sec.  16]  HEAT   OF  FUSION 

ergy  can  be  explained  in  a  general  way  by  the  molecular 
theory. 

The  Latent  Heat  of  Fusion.  —  We  have  said  that  the 
molecules  of  a  true  solid  body  are  unable  to  alter  their 
relative  positions  and  hence  vibrate  ceaselessly  about 
the  same  centers.  In  the  light  of  recent  studies  of  the 
structure  of  crystals,  it  seems  probable  that  the  mole- 
cules of  a  solid  are  also  not  free  to  rotate,  and  that  in  a 
single  crystalline  unit  they  all  point  in  the  same  general 
direction.  This  fixity  of  the  molecules  is  what  gives  the 
solid  its  rigidity,  and  we  must  suppose  it  to  be  due  to 
the  existence  of  definite  forces  of  attraction  existing  be- 
tween them. 

Any  addition  to  the  heat  of  a  body  occurs  in  opposition 
to  these  affinities.  Melting  occurs  when  the  energy  of 
motion  acquired  by  the  molecules  is  sufficient  to  enable 
them  to  break  away  from  the  chains  of  attraction  which 
bind  them  to  their  neighbors.  When  this  happens,  how- 
ever, the  attraction  necessarily  reduces  the  initial  speeds 
of  the  molecules,  because  they  constantly  tend  to  be 
dragged  back  to  then*  original  positions.  Since  heat 
consists  in  the  energy  of  molecular  motion,  this  slowing 
down  of  the  molecules  as  they  escape  from  their  neigh- 
bors must  mean  the  disappearance  or  " absorption"  of 
a  certain  amount  of  heat,  and  this  is  the  latent  heat  of 
fusion  which  we  have  mentioned  above. 

It  is  obvious  that  when  the  temperature  of  a  body  is 
near  the  melting  point  only  a  relatively  small  force  should 
be  required  to  distort  it,  since  the  forces  which  hold  the 
molecules  hi  their  places  are  very  much  weakened.  This 
consideration  explains  the  increased  "malleability"  and 
plasticity  which  is  exhibited  by  many  bodies  at  high 
temperatures. 

When  a  liquid  cools,  the  forces  of  attraction  again 
[103] 


LATENT   HEATS  [Sec.  16 

come  into  play  and  as  the  molecules  drop  into  their  posi- 
tions these  forces  increase  the  velocity  of  their  motion, 
so  that  the  latent  heat  once  more  appears  in  active  form. 

Surface  Tension. — The  molecules  of  a  liquid  are  free 
to  move  anywhere  within  the  liquid  but  are,  for  the  most 
part,  held  within  its  bounds  by  a  force  which  may  be 
appropriately  designated  as  the  "  attraction  of  the  mass." 
The  fact  that  an  attraction  of  this  character  exists  is 
proven  by  the  phenomenon  of  surface  tension.  All  liquids 
behave  as  if  they  were  surrounded  by  a  very  thin  and 
tightly  stretched  skin,  an  effect  which  is  due  to  the  strong 
attraction  exerted  upon  the  surface  molecules  by  those 
which  are  underneath. 

The  Latent  Heat  of  Vaporization.  —  The  evaporation 
of  a  liquid  consists  in  the  escape  of  certain  of  its  mole- 
cules through  this  surface  film.  In  order  to  escape  in  this 
way  they  must  move  at  a  velocity  sufficient  to  enable  them 
to  overcome  the  inwardly  directed  force  which  exists 
at  the  surface.  This  means  that,  in  general,  only  the 
fastest  moving  molecules  can  gain  their  freedom,  and 
even  they  achieve  it  at  a  certain  price,  namely  a  reduc- 
tion of  their  speed. 

The  action  whereby  the  fastest  moving  molecules  are 
constantly  being  removed  from  the  liquid  together  with 
this  reduction  of  speed  necessarily  involves  a  loss  of  heat 
energy.  This  loss  or  absorption  of  heat  is  called  the 
latent  heat  of  vaporization.  Everyone  is  familiar  with  the 
truth  that  the  evaporation  of  a  liquid  has  a  cooling  effect, 
and  this  effect  is  to  be  attributed  to  the  fact  that  evapora- 
tion involves  the  absorption  of  heat  energy. 

When  a  vapor  molecule  returns  to  the  liquid  from  which 
it  sprang,  its  speed  is  increased  as  it  passes  through  the 
surface.  In  this  way  the  latent  heat  of  vaporization  is 
reconverted  into  energy  of  molecular  motion. 

[104] 


Sec.  17]  BOILING   POINTS 

REFERENCES 

Concerning  latent  heats  consult  H.  C.  Jones*  "The  Elements 
of  Physical  Chemistry"  (1902),  pp.  104-106,  161-162,  which, 
however,  does  not  deal  with  the  molecular  explanation  of  the 
phenomena.  This  is  discussed,  mathematically,  hi  W.  Nernst's 
"Theoretical  Chemistry"  (1911),  pp.  236-238.  An  elaborate  dis- 
cussion of  surface  tension  phenomena  will  be  found  in  the  eleventh 
edition  of  the  Encyclopaedia  Britannica  under  "Capillarity." 

Section  17 
"CRITICAL"  AND  BOILING  POINTS  OF  LIQUIDS 

As  the  temperature  of  a  liquid  is  increased,  a  point  is 
finally  reached  at  which  the  influences  of  separation  due 
to  the  movement  of  its  molecules  just  balance  the  forces 
of  inter-molecular  attraction.  This  is  called  the  "  critical 
point"  of  the  liquid. 

Since  the  film  of  surface  tension  which  surrounds  a 
liquid  is  due  to  the  activity  of  its  internal  forces  of  attrac- 
tion, this  film  vanishes  at  the  critical  point,  thus  obliter- 
ating the  distinction  between  the  liquid  and  its  vapor. 
Just  below  the  critical  point  the  latent  heat  of  vaporiza- 
tion is  practically  zero,  on  account  of  the  absence  of  any 
effective  forces  of  attraction  to  be  overcome  hi  separating 
the  molecules.  Both  the  surface  tension  and  the  latent 
heat  of  vaporization  decrease  gradually  as  the  tempera- 
ture rises. 

Under  ordinary  conditions  the  majority  of  liquids  boil 
at  temperatures  which  are  far  below  their  critical  points. 

Since  the  atmosphere  above  a  liquid  exerts  a  confin- 
ing pressure  upon  it,  it  is  impossible  for  vapor  to  form 
within  the  mass  of  the  liquid  until  the  pressure  exerted  by 
the  vapor  itself  is  greater  than  that  of  the  atmosphere. 
When  this  point  has  been  reached  the  production  of  vapor 
within  the  body  of  the  liquid  results  in  the  formation  of 

[105] 


LAWS   OF  GASES  [Sec.  18 

bubbles  which  rise  to  the  surface  and  break,  the  familiar 
phenomenon  of  boiling.  In  accordance  with  this  explana- 
tion it  is  easy  to  see  why  the  boiling  points  of  all  liquids 
should  be  lowered  by  a  decrease  in  the  pressure  of  the 
surrounding  atmosphere,  and  raised  by  an  increase  in 
the  pressure. 

REFERENCES 

On  critical  and  boiling  points  see  A.  D.  Risteen's  "Molecules 
and  the  Molecular  Theory  of  Matter"  (1895),  pp.  80-84. 
Also  Nernst's  "Theoretical  Chemistry"  (1911),  pp.  63-67. 


Section  18 
THE  SIMPLE  LAWS  OF  GASES  AND   OF  SOLUTIONS 

Boyle s  Law.  —  The  relations  which  exist  between 
the  pressure  which  is  exerted  by  a  gas,  its  temperature, 
and  its  state  of  compression,  i.e.,  its  density,  are  very 
simple,  and  are  at  once  accounted  for  by  the  molecular 
theory.  The  pressure  acting  upon  a  vessel  which  con- 
tains a  gas  must  obviously  become  greater  when  the  size 
of  the  vessel  is  decreased,  because  although  the  number 
of  molecules  remains  constant,  the  frequency  with  which 
they  strike  the  sides  of  the  vessel  must  increase.  This  is 
the  basis  of  the  well-known  law  of  Boyle,  which  states 
that  the  pressure  exerted  by  a  given  quantity  of  gas  is 
inversely  proportional  to  the  volume  which  it  occupies. 
The  law  can  in  fact  be  derived  mathematically  by  con- 
sidering the  action  of  the  moving  molecules. 

Charles  Law.  —  When  the  temperature  of  a  gas  in- 
creases, the  molecules  move  faster,  and  hence  the  pres- 
sure must  increase  also.  It  can  be  shown  by  a  simple 
calculation  that  the  pressure  caused  by  the  bombardment 
of  the  sides  of  the  vessel  by  the  flying  molecules  should 
be  proportional  to  the  average  energy  of  this  motion.  This 

[106] 


Sec.  18]  ABSOLUTE   ZERO 

means  that  the  pressure  of  a  gas  is  directly  proportional 
to  its  temperature  measured  in  degrees  above  the  "  ab- 
solute zero":  the  law  of  Charles. 

"  Absolute  Zero."  —  " Absolute  zero"  is  defined  as  a 
point  of  temperature  at  which  the  molecules  are  motion- 
less and  hence  as  a  point  at  which  the  gas-pressure  is  also 
zero.  By  measurements  upon  gases  we  can  find  out  how 
much  their  pressures  decrease  for  each  unit  of  tempera- 
ture, and  if  we  then  divide  their  pressure  at  any  tem- 
perature by  this  amount,  we  shall  learn  the  number  of 
degrees  which  must  be  subtracted  from  the  temperature 
in  question  in  order  to  give  us  the  absolute  zero.  This 
is  the  principle  of  Gay-Lussac.  As  should  be  expected, 
it  turns  out  that  the  change  in  pressure  for  a  given 
change  in  temperature  is  approximately  the  same  pro- 
portion of  the  total  pressure  for  all  gases  studied  under 
the  same  conditions. 

The  Principle  of  Avogadro.  —  In  Section  12  we  have 
discussed  the  principle  according  to  which  all  species  of 
molecules  at  the  same  temperature  have  the  same  aver- 
age energy  of  motion,  regardless  of  their  other  char- 
acteristics. If  this  principle  is  valid  the  pressure  exerted 
by  a  given  body  of  gas  should  be  independent  of  the  kind 
of  molecules  of  which  it  is  made  up,  and  should  depend, 
as  we  have  stated  above,  merely  upon  the  number  of 
molecules  of  any  sort  whatever  which  are  present.  From 
this  it  follows  that  if  the  same  vessel  is  filled  successively 
with  different  kinds  of  gas  at  the  same  temperature  and 
pressure,  the  same  number  of  molecules  will  be  present 
hi  each  case,  or  in  other  words:  " equal  volumes  of  all 
gases  under  the  same  conditions  of  temperature  and 
pressure  contain  equal  numbers  of  molecules,"  —  which 
is  the  famous  principle  of  Avogadro. 

As  we  have  already  seen  (Section  5,  above),  it  follows 
[107] 


OSMOTIC   PRESSURE  [Sec.  19 

from  Avogadro's  rule  that  equal  volumes  of  similarly  con- 
ditioned gases  should  have  weights  which  are  in  the  same 
proportion  as  the  weights  of  their  respective  molecules. 
This  is  another  consideration  which  affects  the  form  of 
the  so-called  "gas  law,"  a  familiar  formula  which  sum- 
marizes the  relationships  which  we  are  now  discussing. 

The  Formula  of  van  der  Waals.  —  It  has  been  shown  by 
experiment  that  the  law  of  Boyle  does  not  hold  for  high 
pressures  and  low  temperatures.  The  reason  for  this 
we  have  already  mentioned  in  Section  2.  It  is  to  be 
found  in  the  fact  that  Boyle's  law  is  calculated  on  the 
assumption  that  the  molecules  are  geometrical  points, 
whereas  in  reality  they  have  a  volume  of  their  own,  a 
fact  which  must  affect  the  ease  with  which  the  gas  is 
compressed.  Moreover  the  gas  molecules  exert  an  at- 
traction upon  each  other  which  tends  to  make  compres- 
sion easier  at  high  than  at  low  pressures.  The  influence 
of  these  factors  has  been  summed  up  in  the  very  accurate 
gas  formula  of  van  der  Waals. 

As  pointed  out  in  Section  19,  below,  the  above  con- 
siderations apply,  at  least  approximately,  to  substances 
in  the  dissolved,  as  well  as  in  the  gaseous  state. 

REFERENCES 

Concerning  the  laws  of  gases  consult  A.  D.  Risteen's  "  Mole- 
cules and  the  Molecular  Theory,"  pp.  40-58;  W.  Nernst's 
11  Theoretical  Chemistry"  (1911),  pp.  198-201,  or  G.  Senter's 
"  Outline  of  Physical  Chemistry"  (1908),  Chapter  H. 

Section  19 
OSMOTIC  PRESSURE 

When  a  substance,  such  as  sugar,  is  dissolved  in  (say) 
water,  its  molecules  are  separated  from  each  other  and 
wander  about  among  the  water  molecules.  If  we  neglect 

[  108  ] 


Sec.  20]  HEAT   CONDUCTION 

the  presence  of  the  latter  we  may  regard  the  state  of  the 
sugar  as  essentially  that  of  a  gas  having  a  density  corre- 
sponding to  the  concentration  of  the  sugar  in  the  water. 

Now  there  is  an  arrangement  by  means  of  which  it  can 
be  shown  that  a  dissolved  substance  actually  does  behave 
like  a  gas.  Suppose  that  some  sugar  solution  is  placed  in 
a  balloon  made  of  a  membrane  through  which  water  can 
pass  without  difficulty,  but  which  is  impenetrable  to  the 
molecules  of  sugar.  If  we  now  place  this  balloon  in  a 
glass  of  water  it  will  tend  to  expand  and  may  even  burst. 
This  effect  is  due  to  the  fact  that  the  sugar  molecules  hi 
the  course  of  then*  heat  vibrations  strike  the  sides  of 
the  balloon,  and  being  unable  to  pass  through  it  as  the 
water  molecules  do,  they  tend  by  their  impact  to  force 
it  outwards. 

General  considerations  lead  us  to  believe  that  the  laws 
of  this  so-called  " osmotic  pressure"  should  be  the  same 
as  those  of  gases,  and  this  is  shown  by  measurements  to 
be  approximately  the  case. 

REFERENCES 

A  popular  discussion  of  the  phenomena  of  osmotic  pressure 
appears  in  W.  C.  D.  Whetham's  "The  Recent  Development  of 
Physical  Science"  (1904),  pp.  104-124.  For  more  advanced  con- 
siderations, see  H.  C.  Jones'  "The  Elements  of  Physical 
Chemistry"  (1902),  pp.  179-199. 

Section  20 
HEAT  CONDUCTION 

Evidently,  if  the  molecular  theory  is  true,  gases  should 
be  better  conductors  of  heat,  per  unit  of  mass,  than  are 
liquids,  since  their  constituent  molecules  are  more  free 
to  change  their  positions,  thus  permitting  a  more  rapid 
mixing  of  the  fast  and  the  slow.  For  the  same  reason 

C109] 


BROWNIAN   MOVEMENT  [Sec.  21 

liquids  should  be  better  conductors  than  solids.  These 
expectations  appear  to  be  borne  out  by  the  facts  of  nature. 
There  seems  to  be  an  exception  to  the  rule,  however, 
in  the  case  of  metals,  which  are  the  best  conductors  of 
heat  known.  This  apparent  exception  is  explained,  how- 
ever, by  the  fact  that  metals  contain  vast  numbers  of 
u  free  electrons,"  particles  so  small  that  they  can  travel 
about  among  the  molecules  of  the  metal  almost  as  if 
they  were  in  unobstructed  space.  Indeed,  the  metal  may 
be  said  to  contain  negative  electricity  in  gaseous  form. 
These  minute  particles  partake  of  the  heat  vibration  of 
the  molecules,  and  by  their  very  rapid  motion  quickly 
bring  the  temperature  of  all  parts  of  a  metallic  body  to  a 
uniform  level.  Metals  are  as  good  conductors  as  the 
lightest  gases  because  the  mass  of  an  electron  is  exceed- 
ingly small,  even  as  compared  with  that  of  the  hydrogen 
atom,  and  hence,  in  accordance  with  the  equipartition 
principle  (see  Section  12)  must  move  faster  at  the  same 
temperature.  Besides  this,  it  is  probable  that  electrons 
can  pass  through  the  body  of  an  atom  without  being 
stopped. 

REFERENCES 

On  the  conduction  of  heat  see  Meyer's  "The  Kinetic  Theory 
of  Gases,"  Part  H,  Chapter  IX  (trans.  1899). 

Section  21 
THE  BROWNIAN   MOVEMENT  AND   ITS   MEASUREMENT 

Perrin's  Experiments.  —  The  work  upon  the  physics  of 
the  Brownian  movement  is  still  new.  Credit  for  the  ex- 
perimental side  of  the  investigation  belongs  largely  to  the 
French  scientist  Jean  Perrin,  whose  methods  of  measur- 
ing the  movements  are  exceedingly  ingenious. 

The  Brownian  particles  which  were  employed  by 
Perrin  were  produced  by  the  precipitation  of  alcoholic 

[110] 


Sec.  21]  BROWNIAN   MOVEMENT 

solutions  of  various  gums  by  pouring  these  solutions  into 
water.  They  varied  in  size  from  about  one  twenty-five- 
thousandth  to  one  two-hundred-and-fiftieth  of  an  inch  in 
diameter.  Their  motions  were  measured  by  direct  ob- 
servation under  the  microscope,  and  also  indirectly  by 
means  of  certain  calculations. 

Perrin  was  able  to  show  that  these  minute  drops  of 
gamboge  and  other  gums,  when  made  into  emulsions  with 
water,  behave  in  every  way  like  the  molecules  of  a  gas 
of  enormous  molecular  weight.  This  applies  to  such 
characteristic  processes  as  average  speed,  rates  of  dif- 
fusion, pressure,  etc. 

Verification  of  Equipartition  of  Energy.  —  One  very 
interesting  outcome  of  Perrin's  work  lies  in  its  remark- 
able verification  of  the  principle  that  the  average  energy 
of  vibration  of  any  species  of  particle  depends  only  on  the 
temperature  of  the  mass  of  matter  considered  and  not 
on  the  weight  or  size  of  the  particles  themselves  (Sec- 
tion 12).  Some  of  the  particles  which  he  studied  were 
many  thousand  times  as  large  and  heavy  as  the  heaviest 
known  atom,  and  yet  their  average  energy  appeared  to 
be  substantially  identical  with  that  characteristic  of  all 
atoms  or  molecules  at  the  temperature  under  observa- 
tion. He  was  able  to  prove,  moreover,  that  the  particles 
have  an  average  energy  of  rotation  which  is  the  same  as 
that  of  their  translatory  movements,  a  result  in  harmony 
with  the  demands  of  theory. 

Other  investigations  have  shown  that  the  Brownian 
movement  in  gases  follows  the  same  laws  as  those  which 
hold  among  particles  suspended  in  liquids. 

REFERENCES 

Perrin's  own  account  of  his  researches  on  the  Brownian  move- 
ment will  be  found  in  "The  Brownian  Movement  and  Molecular 
Reality,"  translated  by  F.  Soddy  (London,  1910).  For  the  most 
part  the  book  is  not  difficult  reading. 

[HI] 


SOLIDS  AND   CRYSTALS  [Sec.  22 

Section  22 
THE  SOLID  AND   CRYSTALLINE   STATES 

The  Crystal  as  a  Unit  of  Structure.  —  The  statement 
that  solid  bodies  are  characterized  by  an  orderly  arrange- 
ment of  their  molecules  implies  that  all  solids  are  crys- 
talline. This  implication  is  probably  correct  hi  spite  of 
the  fact  that  chemistry  distinguishes  between  crystalline 
and  the  so-called  " amorphous"  solids.  Amorphous,  or 
formless,  bodies  may  be  regarded  as  crystalline  bodies 
in  which  the  crystals  are  very  small.1  Strong  reasons 
exist  for  believing  that  this  holds  even  for  the  so-called 
"colloidal"  substances,  which  are  usually  contrasted 
with  "crystalloids." 

If  this  is  the  true  view  the  arrangement  of  the  mole- 
cules in  the  entire  mass  cannot  be  quite  orderly,  but  it 
is  probably  permanent.  When  solids  are  broken  the 
surfaces  of  fracture  generally  coincide  with  the  surfaces 
of  the  crystals  of  which  they  are  composed.  It  would  not 
be  wrong  to  regard  an  ordinary  solid  body  as  a  closely 
packed  mass  of  smaller  bodies,  the  individual  crystals, 
which  alone  represent  the  characteristic  form  of  a  solid. 
The  crystal  thus  becomes  the  unit  of  solid  matter  next 
in  order  above  the  molecule.  However,  there  are  some 
perfectly  definite  crystals  which  seem  to  be  decompos- 
able into  smaller,  similar,  crystals  without  limit.  Such 
a  substance,  for  example,  is  mica,  which  crystallizes  hi 
sheets,  but  no  matter  how  thin  a  sheet  of  mica  may  be, 

1  As  elsewhere  stated,  the  recently  discovered  "  liquid  crystals  " 
introduce  a  somewhat  mysterious  element  into  these  considera- 
tions. It  seems  probable  that  in  the  last  analysis  liquid  crystals 
will  be  found  to  depend  on  a  somewhat  different  set  of  forces 
than  do  crystals  of  solids. 

[112] 


Sec.  22]  CRYSTALS   AND    X  RAYS 

it  is  always  theoretically  possible  to  split  it  into  two 
thinner  sheets,  provided,  of  course,  that  the  first  one  is 
not  of  molecular  thinness.  It  seems  to  be  characteristic 
of  the  molecules  which  compose  the  mica  to  arrange 
themselves  in  geometrical  planes. 

Crystal  Structure  as  Studied  by  X  Rays.  —  Very  re- 
cently it  has  been  found  that  X  rays,  when  reflected  from 
a  crystal  surface,  are  broken  up  into  a  pattern  the  nature 
of  which  varies  with  the  crystal  employed.  The  principle 
in  accordance  with  which  this  pattern  is  formed  is  well 
known,  —  being  that  of  optical  diffraction  or  interference 
—  so  that  from  the  character  of  the  pattern  it  is  possible 
to  deduce  the  arrangement  of  the  molecules  within  the 
crystal.  It  appears  from  these  studies,  the  basis  of  which 
will  be  further  discussed  in  Section  55,  below,  that  the 
unit  of  crystalline  structure,  —  from  a  geometrical  point 
of  view,  at  least  —  is  not  the  molecule,  but  the  atom. 
Crystal  form  appears  to  result  from  an  extension  of  the 
same  architectural  principles  upon  which  the  molecule 
is  built. 

The  cubical  crystal  of  potassium  chloride,  for  example, 
seems  to  be  made  up  of  a  rectangular  lattice-work  of 
alternate  potassium  and  chlorine  atoms,  placed  at  equal 
distances  from  one  another.  It  is  natural  that  there 
should  be  a  close  similarity  between  the  external  shape 
of  the  crystal  and  that  of  the  spatial  configuration  of  its 
component  atoms,  but  it  is  not  always  possible  to  infer 
the  latter  from  the  former.  For  instance,  the  crystal  of 
potassium  bromide,  which  is  externally  similar  to  that 
of  the  chloride,  appears  to  have  atoms  not  only  at  the 
corners  of  a  simple  cubical  lattice-work,  but  also  in  the 
centers  of  all  of  the  cube  faces.  Sodium  chloride,  or 
common  salt,  another  cubical  crystal,  has  an  even  more 
complex  structure. 

[113] 


SOLIDS  AND   CRYSTALS  [Sec.  22 

Stages  Between  Solid  and  Liquid:  Liquid  Crystals.  — 
Of  course  all  conceivable  stages  exist  between  the  solid 
and  the  liquid  states.  It  would  be  difficult,  for  example, 
to  say  whether  such  a  substance  as  asphalt  under  certain 
conditions  of  temperature  is  a  solid  or  a  liquid,  and  ex- 
periments have  shown  that  even  very  hard  and  brittle 
substances  like  the  crystals  of  common  salt  exhibit  defi- 
nite evidence  of  vaporization,  a  process  usually  ascribed 
only  to  liquids. 

Liquid  crystals  probably  depend  upon  the  definite 
relative  arrangement  of  molecules  which  may  neverthe- 
less alter  then*  absolute  position.  Just  as  hi  the  living 
organism,  the  actual  matter  changes  as  time  goes  on, 
although  the  form  remains  practically  constant.  In  other 
words,  it  is  not  impossible  to  conceive  a  combination  of 
orderly  arrangement,  such  as  is  demanded  by  the  crystal- 
line state,  with  relative  freedom  of  translatory  movement 
of  the  molecules  among  themselves,  which  seems  to 
characterize  the  liquid  state.  In  ideal  solids,  however, 
whatever  the  arrangement,  this  movement  cannot  oc- 
cur, and  motion  of  the  molecules  must  be  exclusively 
vibratory. 

It  is  to  be  expected  that  the  new  X  ray  method  of 
studying  crystalline  structure  will  eventually  clear  up 
most  of  these  mysteries,  and  at  the  same  time  throw  light 
upon  the  very  closely  related  problem  of  the  constitution 
of  the  molecule.  As  yet,  only  a  few  simple  crystals  have 
been  analyzed  by  this  means. 

REFERENCES 

On  the  "Molecular  Theory  of  Solids"  see  Risteen's  " Mole- 
cules and  the  Molecular  Theory  of  Matter,"  Chapter  IV. 

A  German  work  on  liquid  crystals  is:  "Die  Neue  Welt  der 
Flussigen  Kristalle,"  by  O.  Lehmann  (1911). 

[114] 


Sec.  23]  DISTRIBUTION   CURVE 

On  the  analysis  of  crystals  by  means  of  X  rays,  see  G.  W.  C. 
Kaye's  "Xrays"  (1914),  pp.  168-204;  and  W.  H.  and  W.  L. 
Bragg's  "X  Rays  and  Crystal  Structure"  (1915). 


Section  23 

VAPOR  PRESSURE  AND  THE  LAW  OF  DISTRIBUTION 
OF  MOLECULAR  SPEEDS 

The  "Distribution  Curve"  of  Molecular  Speeds.  —  In 
Section  11  it  is  said  that  the  behavior  of  molecules 
can  be  studied  satisfactorily  only  by  the  use  of  the  sta- 
tistical method.  Even  at  a  constant  temperature  all  of 
the  molecules  do  not  move  at  the  same  speed;  it  is  the 
average  speed  which  is  constant.  However,  because  of 
the  enormous  number  of  molecules  which  are  contained 
in  any  body  which  we  may  consider,  it  is  possible  to  make 
true  statistical  statements  which  give  us  more  detailed 
information  about  the  state  of  affairs  hi  the  body  than 
does  the  mere  knowledge  of  the  average  velocity  of  the 
molecules. 

Some  of  the  molecules  of  a  body  move  faster  than  the 
average  and  others  move  more  slowly,  but  on  account 
of  the  vast  number  which  are  present  and  the  consequent 
great  frequency  of  their  collisions  there  exists  a  constant 
levelling  tendency,  a  continuous  redistribution  of  energy, 
which  tends  to  make  them  all  approximate  the  average 
speed.  Theoretical  considerations  show  that  in  a  chaos 
of  molecules  such  as  is  contemplated  by  the  kinetic  theory 
there  must  be  far  more  molecules  moving  at  approximately 
the  average  speed  than  at  any  other  speed,  and  that  the  more 
the  speed  of  a  molecule  departs  from  the  average  the 
fewer  of  its  kind  there  must  be.  This  principle  is  often 
spoken  of  as  the  "a  law  of  distribution  of  molecular 
speeds,"  and  it  is  of  the  utmost  importance  in  the  study 

[116] 


VAPOR  PRESSURE 


[Sec.  23 


of  heat  and  allied  phenomena.  As  shown  in  Figure  17, 
it  is  mathematically  similar  to  the  well-known  "  curve 
of  chance  "  with  which  the  reader  may  be  familiar. 

The  Laws  of  "Vapor  Pressure"  •  —  Evaporation,  it  has 
been  explained,  consists  in  the  escape  from  the  surface 


Fig.  17 

"DISTRIBUTION   CURVE"   FOR  MOLECULAR   SPEEDS 

This  curve  shows  geometrically  the  relative  number  of  molecules  mov- 
ing at  speeds  which  differ  more  or  less  from  the  average  speed.  Relative 
speed  is  measured  from  left  to  right  along  the  horizontal  line  and  relative 
number  along  the  vertical  line.  A  is  the  point  corresponding  with  the 
"  average  energy  "  of  all  of  the  molecules.  It  is  evident  from  the  dia- 
gram that  there  are  more  molecules  moving  at  approximately  this  speed 
than  at  any  other,  and  that  the  more  any  (approximate)  speed  differs  from 
the  average  the  fewer  will  be  the  molecules  moving  at  this  speed.  This 
curve  is  a  special  case  of  the  so-called  "curve  of  error"  which  represents 
a  law  of  the  utmost  importance  in  modern  physics. 

of  a  liquid  of  certain  molecules  which  move  faster  than 
the  average.     According  to  the  above  law  of  the  dis- 
tribution of  molecular  velocities,  as  the  temperature  of 
the  liquid  is  increased  the  number  of  molecules  which 
escape  should  increase  also,  and  the  exact  nature  of  this 
increase  can  be  predicted  from  the  law. 
The  vapor  above  a  liquid  is  a  gas,  and  hence  exerts  a 
[116] 


Sec.  23]  VAPOR  PRESSURE 

pressure  upon  the  bodies  immersed  in  it.  This  pressure 
must  follow  the  ordinary  gas  laws  which  we  have  dis- 
cussed in  Section  18.  Since  according  to  these  laws  the 
pressure  is  proportional  to  the  number  of  molecules 
present  in  a  given  volume, and  since  this  number  increases 
with  the  temperature  of  the  liquid,  the  "vapor  pressure," 
as  it  is  called,  should  increase  with  the  temperature  also, 
and  in  a  way  harmonious  with  the  law  of  distribution  of 
molecular  speeds. 

Empirical  measurements  show  that  the  actual  rise  of 
the  vapor  pressure  of  all  liquids  is  quite  closely  in  accord- 
ance with  the  theoretically  deduced  law.  Everybody  is 
acquainted  with  the  general  fact  that  liquids  evaporate 
faster  the  hotter  they  are. 

It  might  at  first  be  thought  that  the  vapor  escaping  from 
a  liquid  would  be  at  a  higher  temperature  than  the  liquid 
itself,  because  only  the  fast-moving  molecules  are  able  to 
pass  through  the  surface.  However,  as  we  have  indicated 
in  Section  16,  the  speeds  of  all  of  these  molecules  are 
slowed  down  under  the  influence  of  the  film  of  surface 
tension.  It  so  happens  that  this  decrease,  which  stands 
for  the  absorption  of  the  latent  heat  of  vaporization  of 
the  substance,  is  of  exactly  the  right  magnitude  to  reduce 
the  average  speed  of  the  escaping  molecules  so  that  the 
temperature  of  the  vapor  is  the  same  as  that  of  the  liquid 
from  which  it  rises.  This  can  be  shown  theoretically  by 
a  consideration  of  the  law  of  distribution  of  molecular 
speeds. 

REFERENCES 

On  the  "law  of  distribution"  refer  to  Meyer's  work,  Chapter 
in  of  Part  I  (see  Note  12). 

A  mathematical  discussion  of  vaporization  will  be  found  in 
Chapter  VII  of  H.  P.  Boynton's  "Applications  of  the  Kinetic  The- 
ory, etc."  (1904). 

[117] 


HEAT  ENERGY  [Sec.  24 

Section  24 
HEAT  ENERGY  AND  SPECIFIC  HEATS 

The  total  heat  energy  of  any  body  is  the  sum  of  the 
energies  of  motion  of  all  of  its  molecules.  It  can  be  esti- 
mated approximately  by  multiplying  the  average  energy 
of  the  molecules  by  the  total  number  of  molecules  in 
the  body. 

When  the  temperature  of  a  given  weight,  say  one 
ounce,  of  a  substance  is  increased  one  degree,  a  definite 
amount  of  energy  has  to  be  added  to  it  in  the  form  of  heat. 
If  this  amount  of  energy  be  divided  by  the  amount  re- 
quired to  bring  about  the  same  change  in  an  equal  weight 
of  water,  the  quotient  is  the  "specific  heat"  of  the  first 
substance. 

Atomic  Heats.  —  Measurements  have  shown  that  the 
specific  heats  of  different  substances  vary  quite  widely, 
but  it  was  discovered  by  the  French  investigators, 
Du  Long  and  Petit,  that  if  the  specific  heat  of  any  ele- 
mentary substance  in  the  solid  state  be  multiplied  by 
its  atomic  weighty  the  resulting  product  is  approximately 
6,  no  matter  what  element  is  taken.  The  reason  for 
this  striking  fact  is  not  far  to  seek.  It  is  found  in  the 
principle  that  the  average  energy  of  motion  of  any  group 
of  molecules  is  independent  of  their  species  (see  Section 
12).  If  all  atoms  at  the  same  temperature  have  the  same 
energy  of  motion,  then  the  energy  which  must  be  added 
to  their  motion  to  produce  a  change  in  temperature  must 
be  independent  of  their  species  and  hence  of  the  nature 
of  the  substance  involved.  Multiplying  the  specific  heat 
of  an  element  by  the  atomic  weight  reduces  the  meas- 
ure of  heat  capacity  to  terms  of  number  of  atoms  alone, 
eliminating  the  factor  of  weight. 

[118] 


Sec.  24]  SPECIFIC   HEATS 

To  raise  the  temperature  of  a  gas  it  is  only  necessary  to 
increase  the  energy  of  motion  of  its  molecules,  but  the 
molecules  of  solids  and  liquids  are  bound  together  by 
strong  forces  of  attraction,  and  when  their  vibrations  are 
increased,  an  amount  of  energy  must  be  utilized  in  over- 
coming these  forces  which  is  equal  to  that  which  enters 
into  the  increased  motion.  Accordingly,  the  specific  heat 
of  a  substance  in  the  solid  or  liquid  form  should  be  about 
twice  that  in  the  gaseous  form,  and  this  is  found  to  be 
the  case  in  nature. 

Now  the  considerations  which  apply  to  elementary 
substances  apply  also  to  compounds,  although  with  less 
accuracy.  When  the  specific  heat  of  a  compound  is  mul- 
tiplied by  its  molecular  weight,  that  is  by  the  sum  of  the 
weights  of  its  contained  atoms,  the  product  does  not 
vary  a  great  deal  from  6,  if  the  substance  is  in  the  solid 
form,  or  3,  if  it  is  in  the  gaseous  form.  However,  the 
variations  which  do  occur  are  sufficient  to  require  explana- 
tion, and  this  must  be  given  in  terms  of  the  "  force  con- 
stitution" of  the  molecule  of  which  we  have  spoken  at 
some  length  in  Section  8.  In  general,  the  stronger  the 
internal  forces  of  the  compound  the  higher  will  be  its 
specific  heat,  since  a  large  amount  of  energy  will  be 
absorbed  in  setting  the  parts  of  its  molecules  into  relative 
vibration,  if  the  attractions  which  hold  these  parts  to- 
gether are  powerful.  It  is  necessary  that  the  atoms 
should  vibrate  within  the  molecule  as  well  as  with  the 
molecule  as  a  whole,  and  the  average  energy  of  this 
internal  vibration  should  be  equal  to  that  of  the  grosser 
molecular  movement. 

It  has  recently  been  discovered  that  the  specific  heats 
of  all  substances  decrease  very  rapidly  at  very  low  tem- 
peratures, so  that  at  absolute  zero  they  would  probably 
themselves  be  zero.  The  exact  meaning  of  this  strange 

[119] 


THE  ELECTRON  [Sec.  25 

fact  is  not  yet  clear,  but  it  seems  to  be  related  with  the 
newly  suspected  atomic  nature  of  radiant,  and  perhaps 
all,  energy.  We  shall  discuss  this  matter  briefly  in  Sec- 
tion 54. 

REFERENCES 

The  specific  heat  of  gases  is  discussed  on  pp.  63-74,  of  Jones' 
" Elements  of  Physical  Chemistry"  (1902),  liquids  on  pages  106- 
110  and  that  of  solids  on  pp.  162-166. 

Consult  also  James  Walker's  "Introduction  to  Physical  Chem- 
istry" (1901),  Chapter  V. 

On  the  changes  in  specific  heats  which  occur  at  low  temperature, 
see  W.  Nernst's  "Theoretical  Chemistry"  (1911),  pp.  710-716. 

Section  25 

THE  DISCOVERY  AND   MEASUREMENT  OF  THE 
ELECTRON 

Thomson  s  Determination  of  the  Electronic  Mass  and 
Charge.  —  The  electron  was  discovered  by  J.  J.  Thomson 
as  the  result  of  a  series  of  epoch-making  and  very  ingen- 
ious experiments.  When  an  electrical  discharge  passes 
through  a  tube  from  which  the  air  has  been  partly  ex- 
hausted it  consists,  hi  part,  of  a  beam  of  rays  which 
seem  to  be  emitted  from  the  negative  pole,  or  "  cathode," 
of  the  battery  or  induction  coil.  (See  Figure  18.) 
Thomson  showed  that  these  so-called  "cathode  rays" 
are  made  up  of  very  minute  bodies  nearly  two  thousand 
times  lighter  than  hydrogen  atoms,  and  moving  at  a 
speed  which  varies  with  conditions  but  which  is  in  gen- 
eral about  one-tenth  that  of  light,  or  about  twenty 
thousand  miles  a  second. 

Thomson  did  not  determine  the  size  of  the  electron 
directly  but  only  its  weight,  or  more  strictly  speaking  its 
mass.  This  he  was  able  to  do  by  use  of  the  well-known 
principle  that  the  more  massive  a  body  is,  and  the  higher 

[120] 


Sec.  25]  CATHODE  RAYS 

its  velocity,  the  greater  is  the  resistance  which  it  offers 
to  change  in  its  state  of  motion.  It  was  found  that  the 
cathode  rays  could  be  bent  by  the  action  of  a  magnet,  a 
fact  which  showed  them  to  bear  electrical  charges,  and 
by  measuring  the  magnitude  of  this  bend  as  compared 
with  the  strength  of  the  magnet  employed,  a  basis  was 
provided  for  the  calculation  of  the  mass  of  the  moving 


Fig.  18 

VACUUM    TUBE    TO    SHOW    THE    ACTION    OF    THE    CATHODE 
RAYS 

The  cathode  rays  are  emitted  from  the  plated  and  travel  away  from  it 
In  straight  lines,  a  fact  which  is  shown  by  the  character  of  the  shadow, 
C,  cast  by  the  metal  cross,  B.  This  shadow  appears  in  the  glow  which  is 
produced  where  the  rays  strike  the  end  of  the  tube. 


particles.  (See  Figure  19.)  Measurement  of  the  bend 
of  the  rays  under  the  influence  of  magnetism  alone  did 
not  permit  Thomson  to  separate  the  effect  of  the  speed  at 
which  the  particles  were  travelling  from  that  due  to  their 
mass,  but  by  studying  the  bend  which  occurred  when 
electrical  as  well  as  magnetic  forces  were  brought  to  bear 
upon  the  rays,  he  was  able  to  obtain  a  measure  which 
depends  upon  the  mass  and  not  upon  the  velocity  of  the 
particles. 

[121] 


THE  ELECTRON 


[Sec.  25 


However,  it  unfortunately  happened  that  these  meas- 
ures were  not  independent  of  the  electrical  charge  borne 

by  the  single  particles, 
and  consequently  he  was 
obliged  to  devise  a 
method  for  the  determi- 
nation of  this  charge. 
The  method  which  he 
actually  used  consisted 
in  a  simple  measurement 
of  the  total  amount  of 
electricity  carried  by  a 
large  number  of  the 
particles,  followed  by  a 
determination  of  the 
number  itself.  From 
these  two  measurements 
the  quantity  of  electricity 
carried  by  one  particle 
could  obviously  be  calcu- 
lated. 
Thomson's  procedure 

It  is  seen  that  after  the  rays  enter  the  large      f°r  finding  the  number  of 
bulb  A  they  move  along  a  circular  path.      particles       Corresponding 

to  a  known  charge  was 
exceedingly  clever.  On 
account  of  its  charge  each 
of  the  particles  is  a  center 
of  forces  of  attraction, 
so  that  if  they  exist  hi 
an  atmosphere  over-sat- 
urated with  moisture, 
this  moisture  tends  to  condense  about  the  individual 
particles,  each  of  the  latter  thus  becoming  the  nucleus 

[122] 


Fig.  19 

HOW  THE   CATHODE   RAYS    MAY 
BE   BENT   BY   A   MAGNET 

This  drawing  represents  a  cross-section 
of  a  tube  which  was  employed  by  J.  J. 
Thomson  in  the  study  of  the  cathode  rays. 


This  is  due  to  the  presence  of  a  magnet 
which  is  placed  outside  of  the  bulb  in  such 
a  way  that  the  lines  of  magnetic  force 
between  the  opposite  poles  of  the  magnet 
are  perpendicular  to  the  path  of  the  rays. 
The  magnet  is  omitted  from  the  drawing 
so  that  the  path  of  the  rays  can  be  clearly 
shown.  However,  if  it  were  supposed  to  be 
really  absent  it  would  be  necessary  to 
represent  the  rays  as  impinging  on  the 
bulb  at  C  instead  of  at  B,  which  is  an 
electrical  condenser  for  collecting  the 
charge  carried  by  the  rays. 


I  I 


Sec.  25]  ELECTRON   MAGNITUDES 

of  a  small  drop  of  water.  The  size  of  the  drops  of  water 
thus  formed  can  be  determined  by  the  rate  at  which  the 
fog  which  they  compose  settles  under  the  pull  of  gravity. 
Since  it  is  easy  to  measure  the  total  amount  of  water 
which  condenses,  the  number  of  droplets  in  the  fog  can 
be  calculated  from  a  knowledge  of  their  individual  size. 
Since  each  droplet  corresponds  to  a  single  electrical 
particle  the  number  of  droplets  gives  a  measure  of  the 
number  of  such  particles  which  are  present,  and  hence 
permits  the  calculation  of  the  charge  which  they  indi- 
vidually bear. 

Knowing  the  amount  of  electricity  carried  by  each 
particle  in  the  cathode  rays,  it  is  possible  to  separate  the 
effect  of  the  charge  from  that  of  the  mass,  and  hence  to 
ascertain  the  magnitude  of  the  latter.  The  most  refined 
measurements  of  this  sort  show  that  the  cathode  ray 
particles,  which  are  now  called  electrons,  have  a  mass 
which  is  about  one  eighteen-hundredth  that  of  the  lightest 
known  atom,  that  of  hydrogen. 

Sources  of  electrons  other  than  the  cathode  rays  are 
now  available,  and  the  nature  of  electrons  has  been  satis- 
factorily proved  to  be  independent  of  their  source. 

The  Size  and  Shape  of  the  Electron.  —  The  size  of  the 
electrons  is  calculated  by  use  of  the  fundamental  laws 
of  electrical  action.  These  laws  imply  that  electrical 
charges,  even  when  free  from  all  matter  in  the  ordinary 
sense  of  the  word,  possess  one  of  the  most  characteristic 
properties  of  matter,  viz.,  mass  or  inertia.  The  laws  state, 
furthermore,  a  definite  relationship  between  the  charge 
of  an  electrical  particle,  its  volume,  and  its  mass,  such 
that  for  a  given  charge  the  mass  increases  as  the  volume 
decreases.  Since  we  know  the  mass  and  the  charge  of 
the  electron  from  the  measurements  described  above,  it 
is  possible  to  calculate  the  volume.  This  calculation  is 

[123] 


THE  ELECTRON  [Sec.  25 

based  upon  the  assumption  that  the  electron  is  made  up 
of  pure  negative  electricity  and  of  nothing  else,  i.e.,  that 
it  contains  no  "matter,"  as  distinguished  from  electricity. 
Although  it  is  difficult  to  give  a  direct  justification  of  this 
assumption  it  is  nevertheless  in  harmony  with  all  of  the 
analogies  of  the  situation,  and  is  contradicted  by  none  of 
the  facts.  Studies  in  radio-activity  have  shown  that 
electrons  can  pass  straight  through  considerable  thick- 
nesses of  solid  substances  and,  indeed,  through  the  atoms 
themselves.  This  clearly  suggests  that  the  electrons  are 
very  minute,  as  is  indicated,  also,  by  the  calculations. 

The  symmetry  of  structure  of  the  electron  seems  to 
be  borne  witness  to  by  certain  measurements  regarding 
the  manner  hi  which  its  mass  changes  with  its  velocity. 
It  follows  from  the  fundamental  electrical  laws  mentioned 
above  that  the  electronic  mass  will  increase  at  high 
speeds  in  a  way  which  depends  in  its  details  upon  the 
shape  and  also  upon  the  internal  structure  of  the  electron. 
By  calculating  the  masses  which  should  be  effective  at 
different  speeds  for  various  probable  types  of  electronic 
constitution,  and  then  comparing  these  results  with  actual 
measurements  we  can  obtain  some  idea  as  to  the  real  shape 
and  structure  of  the  electron.  At  present  the  electron  is 
believed  to  be  spherical  at  low  speeds,  but  it  is  thought 
that  it  becomes  more  or  less  flattened  hi  the  direction  of 
motion  when  it  moves  at  speeds  approaching  that  of  light. 

REFERENCES 

A  considerable  number  of  popular  discussions  of  the  electron 
and  its  measurement  are  obtainable.  Among  these  may  be  men- 
tioned the  following: 

R.  K.  Duncan's  "The  New  Knowledge"  (1908),  Part  3,  Chapters 
III  to  X,  inclusive. 

Sir  Oliver  Lodge's  "Electrons"  (1907),  Chapters  HI-XIV 
inclusive. 

[124] 


Sec.  26-7]  ELECTRICAL  FORCES 

Harry  C.  Jones'  "The  Electrical  Nature  of  Matter  and  Radio- 
Activity"  (1906),  Chapters  I-III  inclusive. 

E.  E.  Fournier  D'Albe's  "The  Electron  Theory"  (1906),  Chap- 
ter XI. 

Section  26 

THE  IMPORTANCE   OF  ELECTRICAL  FORCES  IN 
NATURE 

The  meaning  of  the  statement  that  "most  of  the 
phenomena  hi  nature  are  due,  hi  the  last  analysis,  to 
electrical  attractions  and  repulsions,"  will  become  clear 
to  the  reader  as  he  proceeds.  Part  of  its  significance 
can  be  grasped  at  the  present  stage  of  the  discussion  if 
one  remembers  what  has  been  said  hi  Section  8  about 
the  dependence  of  the  properties  of  substances  upon 
the  nature  of  their  internal  forces.  There  is  now  little 
doubt  that  these  forces  are  electrical.  Also,  chemical 
action  and  all  electrical  phenomena  clearly  involve  the 
agency  of  electrical  forces.  When  we  come  to  study  the 
question  of  the  constitution  of  the  atom  we  shall  see  that 
the  phenomena  of  radio-activity,  and  the  emission  and 
absorption  of  light,  have  an  electrical  origin. 

REFERENCES 

See  Sir  Oliver  Lodge's  "Electrons"  (1907),  Chapter  XVI. 

Section  27 

THE  REACTIONS   OF  ELECTRONS  AND   CHARGED 
ATOMS 

How  Ions  are  Produced.  —  When  an  electron  is  taken 
from  or  added  to  a  previously  neutral  atom  or  molecule 
the  charged  particle  which  is  thus  formed  is  called  an 
"ion"  and  the  process  is  that  of  " ionization."  Various 
means  of  ionization  are  known.  The  collision  of  molecules 

[125] 


IONS  AND   ELECTRONS  [Sec.  27 

in  the  course  of  their  heat  vibration  may  sometimes  be 
sufficiently  violent  to  knock  electrons  out  of  the  molecules. 
A  more  effective  process  of  a  similar  nature,  however, 
lies  in  the  bombardment  of  a  gas  with  flying  electrons  or 
ions,  which  on  account  of  their  speed  and  the  electrical 
forces  which  they  exert  upon  the  electrons  within  the 
gas  molecules  are  able  in  many  cases  to  bring  about  a 
separation  of  the  two.  C.  T.  R.  Wilson,  by  bringing  about 
the  condensation  of  moisture  on  the  ions  which  are 
formed,  has  been  able  to  obtain  accurate  photographs  of 
the  path  of  the  flying  a  particles  from  radium  through  a 
gas.  A  powerful  electrical  field  —  such  as  that  which' 
exists  between  the  terminals  of  a  sparking  induction 
coil  —  will  cause  ionization.  Light  (including  X  rays), 
being  a  form  of  electrical  energy,  can  also  separate  elec- 
trons from  the  atoms  with  which  they  are  combined. 
Ionization  is  likewise  a  common  accompaniment  of  chem- 
ical action,  and  occurs  in  many  chemical  solutions,  a  fact 
later  to  be  considered  in  greater  detail  (see  Section  29). 

A  definite  amount  of  energy  is  required  to  force  an  elec- 
tron out  of  any  atom,  an  amount  which  varies  only  slightly 
with  the  nature  of  the  atom.  The  differences  which  exist, 
however,  are  sufficiently  constant  to  constitute  charac- 
teristic properties  of  the  elements.  The  energy  of  ioniza- 
tion of  a  substance  can  be  estimated  from  the  intensity 
of  the  electrical  field  needed  to  just  produce  ionization. 

The  Interactions  of  Ions  and  Electrons.  —  As  explained 
in  Part  I  an  "  uncharged  "  atom  contains  a  certain  number 
of  electrons  and  also  positive  electricity,  enough  to  neu- 
tralize exactly  their  negative  charges.  If  an  electron  is 
added  to  the  atom  from  the  outside  there  will  be  more 
negative  electricity  than  positive  and  the  atom  will  have 
a  " negative  charge";  whereas  if  an  electron  is  taken  away 
from  it  there  will  be  more  positive  than  negative  elec- 

[126] 


Sec.  27] 


FORCES  BETWEEN  IONS 


tricity  and  the  atom  will  have  a  "positive  charge." 
(7)  in  Figure  20  shows  two  uncharged  atoms,  (4)  two 
negatively  charged  ones,  and  (5)  two  positively  charged 


REPULSION. 


ELECTRON 


REPULSIOK 


STRONG  ATTRACTION 


ONLY  WHEN  NEAR 

ELECTRON 


Fig.  20 

THE  FORCES   ACTING  BETWEEN   IONS,  ATOMS  AND 

ELECTRONS 

These  diagrams  represent  in  a  symbolic  way  the  forces  which  operate 
between  aggregates  of  electrical  charges,  of  various  degrees  of  complica- 
tion. The  diagrams  are  explained  in  the  text. 

ones.  It  must  be  borne  carefully  in  mind  that  these  are 
not  pictures  of  atoms.  They  are  merely  symbolic  draw- 
ings, the  black  dots  representing  electrons,  and  the 

[127] 


IONS  AND   ELECTRONS  [Sec.  27 

"plus  sign"  representing  the  positive  charge  which  is 
inseparable  from  the  atom. 

It  follows  of  course  from  the  laws  of  electricity,  as  re- 
called to  the  reader  in  Chapter  V  under  the  heading, 
"The  Two  Electricities,"  that  the  following  statements 
are  true : 

(a)  Two  electrons  repel  each  other  [see  (1),  Figure  20]. 

(b)  An    electron    is   repelled  by  a  negatively  charged 
atom  (i.e.,  one  which  has  one  electron  too  many  for 
neutrality  (2). 

(c)  An  electron  is  attracted  toward  a  positively  charged 
atom  (i.e.,  one  which  has  one  electron  too  few  for  neu- 
trality) (3). 

( d)  Two  negatively  charged  atoms  repel  each  other  (4). 

(e)  Two  positively  charged  atoms  repel  each  other  (5). 
(/)  A  positively  charged  atom  and  a  negatively  charged 

atom  attract  each  other  (6). 

There  are  also  two  attractions  of  a  different  kind,  one 
of  which  is  already  familiar  to  the  reader,  which  do  not 
follow  obviously  from  the  fundamental  electrical  laws. 
These  are : 

(g)  All  atoms  attract  each  other,  even  when  they  are 
neutral  (7).  This  is  the  familiar  attraction  considered 
hi  Section  10.  It  explains  the  cohesion  of  solids  and  liquids 
in  spite  of  the  violent  heat  vibration  to  which  their  atoms 
and  molecules  are  subject.  This  attraction,  unlike  the 
common  "electrical"  attraction  (/),  is  effective  only  when 
the  two  atoms  are  very  near  each  other. 

(h)  All  uncharged  atoms  attract  electrons  (8).  This 
force,  like  (g),  is  only  effective  when  the  electron  is  very 
near  the  atom.  When  it  is  near,  however,  the  force  be- 
comes very  great.  At  greater  distances  it  is  very  much 
weaker  than  the  familiar  "electrical"  forces  before 
mentioned. 

[128] 


Sec.  28]  HALL  EFFECT 


REFERENCES 

Concerning  ionization,  consult  "The  Electron  Theory"  by  E.  E. 
Fouraier  d'Albe,  Chapter  IV;  "  Electrons,"  by  Sir  Oliver  Lodge, 
Chapter  VII;  "Modern  Theory  of  Physical  Phenomena"  by 
Augusto  Righi  (1904),  Chapter  IV. 

Section  28 

SOME  EFFECTS  CONNECTED  WITH  THE  ELECTRICAL 
CURRENT 

The  "Hall  Effect."  —It  follows  from  a  fundamental 
law  of  electrical  science  that  when  a  moving  electron 
comes  under  the  influence  of  a  magnet  its  normally 
straight-line  path  will  be  changed  to  a  curve.  Hence  if 
the  conduction  of  electricity  through  solids  actually  con- 
sists in  the  bodily  motion  of  electrons  it  should  be  possi- 
ble to  alter  the  direction  of  an  electric  current  in  a  wire 
by  bringing  a  sufficiently  powerful  magnet  near  it.  Ex- 
periment shows  that  this  can  be  done,  the  phenomenon 
being  commonly  known  as  the  "  Hall  effect."  There  are 
certain  stubborn  difficulties  in  connection  with  the  ex- 
planation of  the  Hall  effect,  because  the  changes  are  some- 
times in  one  direction  and  sometimes  in  the  other,  but 
it  seems  highly  probable  that  the  phenomenon  is  due  to 
electron  deflection. 

The  Nature  of  Electrical  Resistance.  —  Different  metals 
—  and  substances  in  general  —  vary  widely  in  their  so- 
called  electrical  conductivity,  or  to  put  it  the  other  way 
round,  they  offer  varying  degrees  of  "resistance"  to  the 
passage  of  the  electrical  current.  There  are  various 
factors  which  determine  the  electrical  conductivity  or 
resistance  of  substances.  In  the  first  place,  it  should  be 
clear  that  the  more  electrons  a  substance  contains  in  a 
given  volume  the  more  electrons  will  move  forward  when 

[129] 


ELECTRIC   CURRENT  [Sec.  28 

an  electrical  force  is  applied  to  it.  The  "current"  or 
" amperage"  is  merely  the  amount  of  electricity  which 
flows  through  a  certain  portion  of  the  wire  in  a  given  time, 
or,  hi  other  words,  the  number  of  electrons  which  pass 
any  fixed  boundary.  So  it  is  obvious  that  the  more 
electrons  there  are  free  to  move  the  greater  will  be  the 
current  under  a  given  electromotive  force  or  " voltage," 
and  therefore  the  higher  the  conductivity  of  the  sub- 
stance or  the  lower  its  resistance.  Substances  such  as 
hard  rubber  and  porcelain  contain  almost  no  free  elec- 
trons and  hence  are  what  we  call  "non-conductors"  or 
"insulators."  Metals  like  copper  and  silver  contain  a 
large  number  of  free  electrons  and  accordingly  are 
"good  conductors." 

Electrical  and  Thermal  Conductivity.  —  In  Section  20 
the  fact  is  mentioned  that  the  extraordinarily  good  heat 
conductivity  of  metals  is  accounted  for  hi  terms  of  the 
free  electrons  which  they  contain.  If  this  is  the  true 
explanation  it  can  be  shown  to  follow  that,  other  things 
equal,  those  metals  which  contain  the  largest  number  of 
free  electrons  will  be  the  best  heat  conductors.  But  such 
metals  will  also  be  the  best  conductors  of  electricity,  and 
hence  it  would  appear  that  some  sort  of  proportionality 
should  exist  between  the  power  of  a  substance  to  conduct 
heat  and  its  power  to  conduct  electricity.  Accurate  meas- 
urements and  calculations  show  that  a  relationship  of 
this  kind  holds  in  nature,  and  that  its  quantitative  char- 
acter is  hi  remarkable  accord  with  the  assumptions  of  the 
electronic  and  molecular  theories. 

Besides  the  number  of  free  electrons  in  a  unit  volume 
of  a  substance  there  are  other  factors  which  must  influ- 
ence its  conductivity.  One  of  these  is  the  "mean  free 
path"  of  the  electrons  among  the  molecules  of  the  sub- 
stance (see  Section  13).  Other  things  being  equal,  the 

[130] 


Sec.  29]  ELECTROLYSIS 

farther  an  electron  can  move  without  striking  an  atom  or 
another  electron  the  better  the  substance  will  conduct 
both  electricity  and  heat. 

It  is  interesting  to  note  the  fact  that  the  direction  of 
movement  of  the  electrons  in  a  wire  is  opposite  to  the 
so-called  direction  of  the  current,  for  the  reason  that  the 
latter  is  what  would  be  the  line  of  motion  of  positive 
electricity  if  any  were  moving.  The  electrons,  it  will  be 
remembered,  are  negative.  When  they  move  oppositely 
to  positive  particles  the  two  produce  identical  magnetic 
effects.  If  electrons  had  been  known  when  the  termi- 
nology was  developed,  the  conventional  direction  of  the 
current  would  probably  be  the  reverse  of  that  now  in 
use. 

REFERENCES 

The  following  references  are  to  simple  discussions  of  the  theory 
of  electrical  conduction  in  solids: 

E.  E.  Fournier  d'Albe's  "The  Electron  Theory,"  Chapter  IV, 
Section  4. 

Sir  Oliver  Lodge's  "Electrons,"  Chapter  X. 

D.  F.  Comstock:  "The  Modern  Theory  of  Electric  Conduc- 
tion," in  the  "Transactions  of  the  American  Electro-Chemical 
Society"  (1912),  Volume  XXI,  pp.  41-48. 

A  somewhat  mathematical  and  more  detailed  account  is  given 
by  Sir  J.  J.  Thomson  in  his  "  Corpuscular  Theory  of  Matter  "  (1907), 
Chapters  IV  and  V. 

Section  29 
ELECTRICAL  CONDUCTION  IN  GASES  AND  LIQUIDS 

Conduction  by  Ions;  Electrolysis.  —  Metals  are  not  the 
only  substances  which  are  good  conductors  of  electricity. 
It  is  a  well-known  fact  that  many  solutions  are  excellent 
conductors,  and  in  this  case  the  conduction  involves  the 
motion  not  of  free  electrons  but  of  charged  atoms  or 
"ions."  Some  of  these  moving  atoms  are  negatively 

[131] 


CONDUCTION   IN   GASES  [Sec.  29 

charged,  that  is,  bear  electrons  in  excess  of  their  normal 
number,  while  others  are  positive  and  have  lost  part  of 
their  regular  complement  of  electrons.  The  current 
through  the  liquid  consists  of  negative  atoms  or  ions 
moving  in  one  direction  and  of  positive  atoms  or  ions 
moving  in  the  opposite  direction.  Both  of  these  lose 
then*  charges  when  they  come  into  contact  with  the 
"electrodes"  by  which  the  current  enters  and  leaves 
the  solution,  the  former  at  the  positive  pole  and  the 
latter  at  the  negative  pole.  This  means  that  atoms  of 
the  positively  charged  substances  will  collect  about  the 
negative  pole  while  those  of  the  negatively  charged  kind 
will  segregate  about  the  positive  electrode.  This  process 
is  commonly  called  electrolysis,  and  will  be  further  dis- 
cussed at  another  point.  For  this  type  of  conduction  it  is 
necessary  for  the  liquid  to  contain  ions.  (See  Section  27.) 
The  conduction  of  electricity  through  gases  also  de- 
pends upon  the  presence  of  ions.  Free  electrons,  however, 
are  often  present  and  active.  To  the  study  of  phenomena 
connected  with  the  passage  of  electricity  through  gases 
we  owe  a  great  deal  of  our  knowledge  of  the  nature  of 
electrical  processes  in  general,  since  the  conditions  here 
are  particularly  favorable  for  observation. 

REFERENCES 

On  the  conduction  of  electricity  through  gases  see  W.  C.  D. 
Whetham's  "The  Recent  Development  of  Physical  Science" 
(1904),  Chapter  V. 

Sir  J.  J.  Thomson's  great  work  "The  Conduction  of  Electricity 
Through  Gases"  should  also  be  mentioned. 

On  conduction  in  solutions,  see  Augusto  Righi's  "Modern  The- 
ory of  Physical  Phenomena  (1904),  Chapter  I,  and  "The  Theory 
of  Electrolytic  Dissociation,"  by  Harry  C.  Jones  (1900). 


[132] 


Sec.  30]  ELECTRIC   POWER 

Section  30 
THE  ELECTRICAL  TRANSMISSION  OF  POWER 

The  analogy  between  the  transmission  of  power  by 
electricity  and  by  compressed  air  really  amounts  to 
something  very  close  to  identity,  if  the  modern  view  is 
correct.  When  air  is  pumped  in  at  one  end  of  a  pipe 
the  air  molecules  at  that  end  exert  an  increased  force 
upon  the  others  further  along  and  thus  increase  the  pres- 
sure throughout  the  system.  Similarly,  in  an  electrical 
circuit  an  increase  in  voltage  may  be  thought  of  as  cor- 
responding to  the  introduction  of  further  electrons  into 
that  part  of  the  circuit  which  lies  just  beyond  the  dynamo 
or  battery.  These  repel  neighboring  electrons  and  thus 
the  electrical  pressure  increases  all  along  the  circuit. 

Ordinarily  the  electrons  are  confined  within  the  body 
of  the  wire  just  as  the  air  molecules  are  held  within  the 
pipe.  Leaks,  however,  may  occur  —  as  in  the  case  of 
compressed  air  systems  —  as  shown  by  the  glow  which 
sometimes  surrounds  high  tension  lines  at  night. 

The  influence  of  electrons  upon  each  other's  motions 
of  course  depends  in  large  part  upon  the  enormous  elec- 
trical repulsions  which  exist  between  them.  This  is  not 
so  clearly  the  case  with  molecules. 

REFERENCES 

See  E.  E.  Fournier  d'Albe,  "The  Electron  Theory"  (1906), 
Chapter  VH. 

Section  31 
THERMO-ELECTRICITY 

The  Principle  of  the  Thermopile.  —  Roughly  speaking, 
the  number  of  free  electrons  in  a  substance  is  a  measure 
of  the  lack  of  affinity  of  its  atoms  or  molecules  for  elec- 
trons. The  fact  that  this  aftimty  varies  for  different  sub- 

[133] 


THERMO-ELECTRICITY 


[Sec.  31 


stances  has  some  interesting  consequences  quite  apart 
from  the  production  of  various  degrees  of  electrical  con- 
B  ductivity.  For  example,  if  two 

metals  the  atoms  of  one  of 
which  have  a  greater  affinity 
for  electrons  than  have  those 
of  the  other,  are  placed  in 
contact  the  former  will  appro- 
priate electrons  from  the 
latter.  This  is  due  to  the  fact 
that  the  "  evaporation  of  elec- 
trons" from  the  surface  of 
one  of  the  metals  is  more 
rapid  than  that  from  the  sur- 
face of  the  other  metal,  so 
that  the  first  gives  out  to  the 
second  more  electrons  than 
it  receives.  Hence  the  second 
metal  becomes  negatively 
charged,  while  the  first  ac- 
quires a  positive  charge. 

If  we  suppose  the  two  met- 
als in  question  to  be  in  the 
form  of  horse-shoe  shaped 
wires  touching  each  other  at 
their  extremities  it  can  easily 
be  seen  that  no  current  of 
electricity  will  flow  through 
the  circuit  which  is  thus 
formed,  for  the  reason  that 
the  electrical  forces  which  ex- 
ist at  one  junction  are  exactly 
balanced  by  those  existing  at  the  other  junction.  Suppose, 
however,  that  the  latter  is  heated  to  a  temperature  higher 

[134] 


Fig.  21 

A  THERMO-ELECTRIC  CIRCUIT 
This  is  a  symbolic  drawing.  The 
circle  as  a  whole  represents  the 
complete  electrical  circuit,  the  left 
half  being  composed  of  a  metal 
which  emits  electrons  freely  and 
the  right  half  of  one  which  parts 
with  its  electrons  less  easily.  If 
the  junctions  A  and  E  are  both  at 
the  same  temperature  no  current 
will  flow,  since  the  tendency  to- 
wards a  clockwise  current  which 
exists  at  E  is  exactly  balanced  by 
the  opposite  tendency  existing  at 
A.  However,  when  the  junction  A 
is  heated  these  tendencies  are  no 
longer  exactly  in  equilibrium  and 
electrons  move  around  the  circuit 
in  the  direction  of  the  arrows.  It  is 
not  necessary  that  the  circuit  should 
be  made  up  of  equal  masses  of  only 
two  different  metals.  It  may  be 
broken  at  any  point  and  long  wires 
of  any  sort  of  conducting  substance 
introduced  without  altering  its  gen- 
eral principle. 


Sec.  31]  THERMO-ELECTRIC   SERIES 

than  that  of  the  former.  (See  Figure  21.)  This  will 
bring  about  a  change  in  the  electrical  forces  at  the 
point  which  is  being  heated,  due  in  part  to  the  fact  that 
the  number  of  electrons  thrown  off  by  one  metal  increases 
with  the  temperature  faster  than  the  number  emitted 
by  the  other  metal.  The  electrical  equilibrium  of  the 
circuit  is  thus  disturbed,  and  a  current  will  flow,  that 
is,  electrons  will  move  from  one  junction  towards  the 
other.  This  motion  of  the  electrons  carries  heat  energy 
from  the  hot  to  the  cold  junction  so  that  continued  heat- 
ing and  cooling  is  necessary  in  order  that  it  should  per- 
sist. This  is  the  principle  of  the  so-called  thermopile. 
The  Thermo- Electric  Series  of  the  Metals.  —  By  studying 
different  thermo-electric  circuits  of  the  sort  described 
above  we  can  arrange  a  series  of  metals  in  which  by  con- 
tact with  a  standard  metal,  each  member  of  the  series 
gives  a  higher  voltage  than  the  member  preceding  it. 
It  is  found  that  this  series  has  close  affinities  with 
another  series  which  is  determined  by  studying  the  vol- 
tages generated  by  the  same  metals  in  the  form  of  an 
ordinary  electrical  battery.  The  exact  nature  of  the 
electro-motive  series  varies  with  the  temperature,  on 
account  of  the  fact  that  as  different  metals  are  heated  the 
number  of  free  electrons  which  they  contain  in  a  given 
volume  does  not  necessarily  alter  in  the  same  way.  A 
sample  sequence  is  represented  below: 

THERMO-ELECTRIC   SERIES   OF   METALS 
Metal  Relative  Potential  Difference 

Bismuth' 89  to  97 

Nickel 22 

German-silver 11.75 

Lead 0 

Platinum —    0.9 

Copper -    1.36 

Zinc -    2.3 

Iron -  17.5 

Antimony -  22.6  to  -  26.4 

Tellurium 502. 

Selenium 800. 

Lead  is  taken  as  the  standard  metal. 

[135] 


CHEMICAL  AFFINITY  [Sec.  32 

REFERENCES 

On  thermo-electricity,  consult:  E.  E.  Fournier  d'Albe's  "The 
Electron  Theory"  (1906),  Chapter  V. 

See  also:  O.  W.  Richardson's  " Aggregates  of  Electrons,"  in 
the  " Proceedings  of  the  American  Philosophical  Society"  (1911), 
volume  50,  pp.  347-366,  and 

W.  C.  D.  Whetham's  "The  Theory  of  Experimental  Electricity" 
(1912),  Chapter  VI. 

On  the  evaporation  of  electrons  see  N.  Campbell's  "Modern 
Electrical  Theory,"  second  edition  (1913),  pp.  81-84. 

Section  32 
CHEMICAL  AFFINITY 

Electro-positive  and  Electro-negative  Elements.* —  Chemi- 
cal affinity  may  perhaps  be  regarded  as  another  expres- 
sion of  the  fact  that  different  kinds  of  atoms  have  varying 
degrees  of  attraction  for  electrons.  It  has  been  known 
for  a  long  time  that  the  chemical  elements  could  be 
grouped  into  three  not  very  sharply  defined  classes, 
those  which  were  characteristically  electro-positive,  those 
characteristically  electro-negative,  and  those  which  might 
be  either  electro-positive  or  electro-negative,  according  to 
the  circumstances.  Hydrogen  and  the  metals  are  the 
most  powerfully  electro-positive  of  the  elements,  while 
such  substances  as  oxygen,  chlorine,  fluorine,  etc.,  are 
strongly  electro-negative.  As  we  have  seen,  the  former 
lose  electrons  easily;  the  latter,  on  the  other  hand,  ap- 
propriate with  great  avidity  electrons  not  their  own. 

It  is  clear  that  when  a  neutral  atom  of  hydrogen,  for 
example,  parts  with  an  electron  it  must  become  positively 
charged,  and  if  this  electron  or  another  is  appropriated 
by  an  atom  of  chlorine,  for  instance,  the  latter  becomes 
negatively  charged.  Positively  and  negatively  charged 
atoms  of  this  sort  will  obviously  tend  to  combine,  owing 

[136] 


Sec.  32]  POSITIVE  AND   NEGATIVE  ELEMENTS 

to  the  electrical  attractions  existing  between  them,  and 
these  attractions,  it  seems  probable,  constitute  chemical 
affinity. 

When  we  study  the  actual  constitution  of  chemical  com- 
pounds we  find  that  the  most  common  compounds  are 
made  up  of  just  such  electro-positive  and  electro-negative 
components.  Common  salt,  for  example,  is  composed  of 
the  strongly  positive  element  sodium,  combined  with  the 
strongly  negative  element  chlorine. 

Any  Element  may  be  Positive  or  Negative.  —  However, 
if  we  make  a  table  of  elements  which  we  suppose  to  be 
electro-positive  and  another  of  those  which  we  suppose 
to  be  electro-negative,  we  soon  discover  that,  however 
we  may  construct  the  table,  exceptions  always  occur  to  a 
rule  which  states  that  only  atoms  of  opposite  sign  com- 
bine with  each  other.  For  a  long  tune  it  was  thought  that 
the  fact  that  two  negative  or  two  positive  atoms  could 
combine  chemically  was  a  refutation  of  the  electrical 
theory  of  chemical  affinity,  since  atoms  bearing  charges 
of  like  sign  could  only  repel  each  other.  The  electron 
theory  removes  this  difficulty,  along  with  others,  for  any 
atom  can  become  negatively  charged  if  it  can  gain  an 
electron,  or  positively  charged  if  it  can  lose  one,  and  the 
same  atom  may  conceivably  suffer  either  of  these  changes 
according  to  the  conditions  under  which  it  is  placed.  No 
two  of  the  elements  have  the  same  affinity  for  negative 
electricity,  and  if  any  two  of  them  are  mixed  under  proper 
conditions  the  atoms  of  the  one  having  the  lesser  affinity 
will  lose  electrons  while  the  atoms  of  the  other  will  gain 
them.  Mixed  with  a  different  element,  the  same  atoms 
which  here  gain  electrons  might  lose  them,  if  the  element 
in  question  were  the  more  strongly  electro-negative.  In 
other  words,  we  can  speak  of  the  electro-negativity  or 
positivity  of  the  elements  in  a  relative  sense  only. 

[137] 


CHEMICAL  AFFINITY  [Sec.  32 

There  is  one  difficulty  which  remains  outstanding, 
however,  and  that  is  the  explanation  of  the  chemical 
affinity  which  apparently  exists  between  atoms  of  the 

same  species.    We  have  al- 

-f\     ;'—  ready  mentioned  the  fact  in 

\    /  Section  7  that  almost  all  of 

_•  i  ,  the  elements  form  complex 

molecules  even  in  the  pure 

state.    This  implies  the  exist- 

•  i !_.  ence  of  chemical  affinity  be- 

i  1  tween  atoms  of   the    same 

/    \  kind.     Such  affinity  is  prob- 

— '      \+  ably  to  be  explained  by  the 

idea   that  the  positive   and 

negative  charges  within  any 

WHICH  TWO  NEUTRAL  AGGRE-  atom  are  not  uniformly  dis- 


throughout  it,  and 
OTHER  hence    that  the    surface    of 

The  two  arcs  represented  in  the  figure    an     atom     is,     SO     to    Speak. 
may  be   thought  of   as   cross-sections 
through  the  surfaces  of  two  adjacent    mottled.      TWO    atoms    OI    the 

same  kind  may  thus   stick 

toeether  by  electrical  attrac- 

tive  components  of  the  other.     Taken    tion    if,   as 


as  wholes  both  atoms  are  electrically  ,     j       .         _».  rtrt 

neutral,  but  at  close  range  powerful  represented      Ul     Figure     22, 

mutual  attractions  may  still  exist  be-  thev    afe     gQ     or;ented     With 

tween  their  parts.    This  diagram  must  mev    '  '      ^  Li 

not  be  regarded  as  an  accurate  picture  fCSpect     to    each    other    that 
of  the  surface  of  an  atom. 

the    negative    mottlings    of 

one  coincide  with  the  positive  mottlings  of   the  other, 
and  vice  versa. 

The  "Inert"  Elements.  —  Some  of  the  elements,  such 
as  argon,  helium,  and  the  like,  are  chemically  inert,  that 
is,  they  refuse  to  combine  with  any  other  elements.  The 
reason  for  this  probably  lies  in  the  fact  that  they  are 
incapable  of  becoming  permanently  ionized,  that  is,  they 

[138] 


Sec.  33]  SOLUTION 

have  neither  a  very  strong  tendency  to  gain  additional 
electrons  or  to  part  with  those  which  they  naturally 
possess,  but  rather  tend  to  be  stable  in  this  respect.  We 
have  mentioned  the  fact  in  Section  6  that  the  electrical 
character  of  the  elements  varies  with  other  of  their  prop- 
erties according  to  the  law  of  the  periodic  table.  The 
inert  elements  all  fall  into  the  same  family,  and  each  of 
them  lies  between  a  strongly  electro-negative  element  on 
one  side  and  a  strongly  electro-positive  element  on  the 
other,  so  that  it  is  perhaps  not  surprising  that  they  should 
be  neutral. 

REFERENCES 

See  the  reference,  already  given,  to  Sir  Oliver  Lodge's  "Elec- 
trons," Chapter  XVI,  and  Norman  Campbell's  "  Modern  Electrical 
Theory,"  second  edition  (1913),  Chapter  XIII,  esp.  pp.  340  ff. 

Section  33 
SOLUTION  AND   ELECTRICAL  DECOMPOSITION 

How  Water  "Ionizes"  Dissolved  Substances.  —  It  has 
been  asserted  in  the  course  of  our  discussion  that  the 
forces  of  chemical  affinity  are  probably  electrical.  If 
this  is  true,  any  influence  which  tends  to  weaken  these 
forces  should  cause  the  molecules  to  fall  apart  under 
the  influence  of  then*  heat  vibrations. 

Now  all  substances  possess  a  property  called  dielectric 
capacity,  which  can  easily  be  measured,  but  concerning 
the  exact  nature  of  which  we  need  not  here  trouble  our- 
selves. Suffice  it  to  say  that  the  electrical  forces  between 
any  given  set  of  charged  particles  become  less  as  the 
dielectric  capacity  of  the  medium  in  which  they  are 
placed  becomes  greater.  All  material  bodies  have  a 
higher  dielectric  capacity  than  has  empty  space  and  it 
would  seem  that  if  two  bodies  rightly  selected  could  come 

[139] 


ELECTROLYTIC   DISSOCIATION         [Sec.  33 

into  sufficiently  intimate  contact  with  each  other  the 
large  dielectric  capacity  of  the  one  might  bring  about  a 
separation  of  the  electrical  particles  making  up  the  other. 
Just  this  intimacy  of  contact  is  assured  when  one  of  the 
substances  is  a  liquid  and  the  other  is  in  solution  within 
it.  Suppose,  for  example,  that  a  solid  like  common  salt, 
the  molecules  of  which  are  made  up  of  one  atom  of  chlo- 
rine and  one  atom  of  sodium,  is  dissolved  in  water. 
According  to  our  view  of  chemical  affinity,  these  mole- 
cules are  held  together  by  the  attraction  which  exists 
between  the  positively  charged  sodium  atom,  Na+  and 
the  negatively  charged  chlorine  atom,  Cl~.  If  the  prop- 
erties of  the  water  cause  it  to  weaken  this  attraction  so 
that  the  oppositely  charged  parts  of  the  salt  molecules  can 
be  knocked  apart,  the  solution  will  contain  free  charged 
atoms,  or  ions.  This  process  actually  occurs  with  a  great 
many  substances  which  can  be  dissolved  in  water.  It 
is  called  technically  "electrolytic  dissociation." 

The  Motion  of  Ions  Through  a  Solution.  —  When  so- 
lutions conduct  electricity  the  current  is  due  to  the  bodily 
motion  of  the  ions  which  have  been  produced  by  the 
splitting  up  of  the  molecules  of  the  dissolved  substance. 
We  have  discussed  this  matter  in  Section  29,  and  have 
also  referred  to  some  of  the  related  phenomena  in  Sec- 
tion 2. 

The  electrical  dissociation  of  the  molecules  of  dissolved 
substances,  which  can  easily  be  proven  by  experiment 
to  exist,  furnishes  us  with  another  striking  verification 
of  the  idea  that  chemical  affinity  depends  upon  electrical 
forces. 

The  Effect  of  lonization  on  Boiling  and  Freezing  Points. 
-  When  any  substance  is  dissolved  in  a  liquid,  the  boil- 
ing point  of  the  liquid  is  raised  and  its  freezing  point  is 
lowered  to  an  extent  which  is  approximately  proportional 

[140] 


Sec.  34]  CHEMICAL  VALENCY 

to  the  number  of  molecules  which  have  entered  into 
solution.  The  cause  of  these  changes  can  be  stated  in 
terms  of  the  kinetic  molecular  theory.  The  important 
fact  for  us  to  notice  here,  however,  is  that  when  the 
molecules  of  the  dissolved  substance  are  split  up  into 
ions  the  number  of  effective  molecules  is  thereby  greatly 
increased,  and  that  if  this  is  the  case  the  effect  upon  the 
boiling  and  freezing  points  should  be  greater  than  would 
be  expected  upon  the  assumption  that  no  such  splitting 
of  the  molecules  occurred.  Empirical  measurements  ap- 
pear to  verify  this  conclusion,  and  also  show  the  presence 
of  an  abnormally  high  osmotic  pressure  (see  Section  19) 
in  solutions  of  electrically  dissociated  substances. 

REFERENCES 

Refer  to  Nernst's  "Theoretical  Chemistry"  (1911),  Book  H, 
Chapter  VII,  or  Talbot  and  Blanchard's  "The  Electrolytic  Disso- 
ciation Theory,  etc."  (1907). 

W.  C.  D.  Whetham's  "The  Recent  Development  of  Physical 
Science"  (1904),  Chapter  IV. 

A  complete  book  on  this  subject  is  Whetham's  "Treatise  on 
the  Theory  of  Solutions"  (1902). 

Section  34 
CHEMICAL  VALENCY 

It  is  a  well-known  fact  that  the  chemical  elements 
combine  with  each  other  in  proportions  which  vary  with 
the  element  which  is  considered.  For  example,  one  atom 
of  chlorine  normally  combines  with  only  one  atom  of  hy- 
drogen, while  an  atom  of  oxygen  usually  combines  with 
two  of  hydrogen.  A  single  carbon  atom,  on  the  other 
hand,  will  as  a  rule  unite  with  four  hydrogen  atoms. 
The  number  of  hydrogen  atoms  with  which  an  atom  of  a 
given  element  will  combine  is  called  the  "valency"  of 

[141] 


CHEMICAL  ACTION  [Sec.  35 

the  element.  Some  active  elements  do  not  combine 
readily  with  hydrogen,  but  these  unite  with  oxygen,  the 
valency  or  combining  power  of  which  is  known  to  be  two. 
The  valency  of  an  element  is  not  perfectly  constant, 
but  it  is  nevertheless  fairly  characteristic.  Its  magnitude 
probably  depends  upon  the  affinity  of  the  atoms  of  the 
element  for  electrons.  The  valency  of  an  element  hi  any 
specified  compound  represents  the  number  of  electrons 
which  one  of  its  atoms  has  either  gained  or  lost. 

REFERENCES 

See  R.  K.  Duncan's  "The  New  Knowledge"  (1908),  166-167, 
and  N.  Campbell's  "Modern  Electrical  Theory,"  second  edition 
(1913),  pp.  340-350. 


Section  35 
CHEMICAL  ACTION 

Chemical  action  is  not  as  straightforward  a  process  as 
would  at  first  appear,  since  it  must  depend  upon  the 
random  collision  of  the  reacting  molecules  or  atoms. 

Let  us  consider  carefully  a  case  of  chemical  combina- 
tion or  " synthesis"  and  see  if  we  can  visualize  for  it 
a  reasonable  mechanism. 

The  Basis  of  the  Law  of  "Chemical  Mass  Action."  — 
We  start  with  a  certain  number  of  atoms  of  the  species 
A  and  an  equal  number  of  the  species  B,  which  are 
capable  of  combining  with  each  other  in  the  proportion  of 
one  to  one.  However,  they  cannot  do  this  unless  all  of 
the  individual  A  atoms  come  into  ultimate  contact  with 
as  many  different  B  atoms.  All  of  the  atoms  are  bounding 
about  at  random  and  colliding  with  each  other  like  the 
members  of  a  panic-stricken  crowd,  and  not  every  colli- 
sion that  occurs  is  between  an  A  and  a  B  atom.  This  will 

[142] 


Sec.  35]  MASS   ACTION 

be  especially  true  after  a  sufficient  number  of  favorable 
collisions  have  occurred,  so  that  the  reaction  has  pro- 
gressed some  distance,  for  at  this  time  the  mixture  will 
contain  not  only  separate  A  and  B  atoms  but  also  the  AB 
molecules  which  have  been  formed.  When  these  collide 
with  each  other  and  with  the  atom,  no  union  occurs,  so 
that  as  the  reaction  proceeds  the  number  of  collisions 
favorable  to  chemical  combination  constantly  decreases. 

It  will  be  observed  that  this  decrease  in  the  number 
of  favorable  collisions  is  caused  by  a  diminution  in  the 
number  of  reactable  atoms  which  are  present.  Now  the 
rate  or  "velocity"  of  any  chemical  reaction  consists  in 
the  number  of  molecules  which  are  formed  or  decomposed 
in  a  given  time,  and  this,  in  turn,  must  depend  upon  the 
number  of  collisions  occurring  in  that  time.  From  this 
it  becomes  clear  that  as  a  reaction  proceeds,  its  velocity 
should  constantly  diminish,  and  it  can  be  shown  in  fact 
that  at  any  time  this  velocity  is  proportional  to  the  num- 
ber of  active  atoms  or  molecules  of  each  kind  which  re- 
main. This  is  the  law  of  chemical  mass  action,  which  is  of 
the  utmost  importance  in  the  study  of  chemical  change. 

Chemical  "Equilibrium."  -The  reaction  which  we 
have  considered  above  would  be  complete  when  each  of 
the  A  atoms  had  combined  with  a  separate  B  atom.  It 
would  take  a  long  time  for  such  a  reaction  to  reach  com- 
pletion, for  the  reason  that  when  nearly  all  of  the  atoms 
had  combined,  the  remaining  ones  would  be  separated  by 
a  multitude  of  inert  molecules  and  so  would  have  small 
chance  of  encountering  each  other.  However,  in  reality 
most  chemical  changes  are  reversible,  that  is,  the  mole- 
cules which  are  formed  by  the  reaction  tend  to  break  up 
again  and  reproduce  the  original  substances.  Thus,  most 
reactions  will  consist  hi  two  types  of  change,  a  forward 
and  a  reverse.  When  the  reaction  commences,  the  for- 

[143] 


EFFECTS   OF   CHEMICAL   CHANGE     [Sec.  36 

ward  change  is  the  most  rapid  because  there  are  more 
molecules  to  enter  into  it,  but  as  the  reaction  progresses 
the  products  of  this  change  pile  up,  and  consequently 
necessitate  a  constant  increase  in  the  reverse  reaction. 
There  finally  comes  a  time,  of  course,  when  the  rates  of 
the  two  opposite  changes  are  equal  so  that  they  balance 
each  other.  At  this  time  the  " equilibrium  point"  of  the 
reaction  is  said  to  have  been  reached,  and  apparently  all 
chemical  change  has  ceased.  This  appearance  is  deceiv- 
ing, however,  for  beneath  the  seeming  quiet  there  goes 
on  a  ceaseless  balanced  activity.  It  is  characteristic  of 
the  modern  view  of  things  to  suppose  that  nearly  all 
cases  of  seeming  rest  are  hi  reality  cases  of  balanced 
motions. 

REFERENCES 

On  the  law  of  chemical  mass  action,  see  W.  Nernst's  "Theoret- 
ical Chemistry"  (1911),  Book  II,  Chapter  I. 

Also:  G.  Senter's  "Outline  of  Physical  Chemistry"  (1908), 
Chapter  VH. 

Harry  C.  Jones'  "The  Elements  of  Physical  Chemistry"  (1902), 
Chapter  IX. 

Section  36 
EFFECTS  AND   CONDITIONS  OF  CHEMICAL  CHANGE 

The  Heat  Produced  by  Chemical  Change.  —  It  is  a  well- 
known  fact  that  when  chemical  changes  occur,  energy  is 
usually  liberated  in  one  form  or  another,  most  commonly 
as  heat.  If  atoms  are  to  combine,  under  the  influence  of 
chemical  attraction  they  must  first  move  towards  each 
other  hi  response  to  this  attraction,  and  in  so  doing  they 
must  acquire  energy  of  motion.  Hence,  in  general,  the 
temperature  of  the  reacting  substances  increases  in 
proportion  to  the  strength  of  the  chemical  affinities  which 

are  active. 

[144] 


Sec.  36]    CONDITIONS  OF  CHEMICAL  CHANGE 

The  Generation  of  Light  and  Electric  Current.  —  Chemi- 
cal change  may  produce  electrical  effects  if  the  condi- 
tions are  arranged  as  in  the  ordinary  "battery,"  so  that 
charged  atoms  which  are  free  to  move  can  combine  with 
the  atoms  of  a  solid  electrical  conductor  and  deposit 
their  charges,  so  that  the  conductor  as  a  whole  is  electri- 
fied and  tends  to  become  the  origin  of  an  electrical  cur- 
rent. The  production  of  light  by  chemical  change  may  be 
due  to  the  fact  that  light  is  caused  directly  by  the  vibra- 
tion of  the  electrons  which  are  active  during  the  change, 
or  it  may  be  indirectly  caused  through  the  rise  in  tem- 
perature brought  about  by  the  reaction. 

Conditions  Favor  ing  Chemical  Action. — Almost  all  chem- 
ical changes  involve  not  only  the  formation  of  new 
molecules  but  also  the  breaking  up  of  old  ones,  and  since 
the  decomposition  of  molecules  would  tend  to  be  favored 
by  the  collisions  of  the  molecules  in  their  heat  motion, 
we  should  expect  for  this  reason  alone  that  the  rapidity 
of  a  chemical  change  would  increase  with  the  tempera- 
ture. As  mentioned  in  Part  I,  the  transfer  of  electrons 
from  one  atom  to  another  must  also  take  place  more 
readily  at  high  than  at  low  temperatures,  and  this,  too, 
aids  the  reaction. 

In  the  case  of  reaction  between  gases,  high  pressures 
are  favorable  to  chemical  reaction,  since  the  more  mole- 
cules there  are  in  a  given  volume  the  more  frequently 
they  must  collide  and  hence  the  greater  their  chances  of 
decomposition  and  recombination.  Another  important 
condition  favoring  chemical  change  is  the  presence  of 
a  catalyzer,  which  is  merely  some  foreign  substance 
capable  of  accelerating  a  reaction  without  being  changed 
itself.  The  exact  general  mechanism  by  which  catalysis 
is  produced  is  not  clearly  understood,  but  it  is  probable 
that  the  catalyzer  is  an  ionizing  agent  of  some  sort. 

[145] 


LIGHT   WAVES  [Sec.  37 

"Chemical  energy,"  which  is  utilized  when  coal,  for  ex- 
ample, is  burned  under  the  boiler  of  a  steam  engine,  is 
really  the  energy  of  attraction  of  charged  atoms  or  groups 
of  atoms.  Human  life  and  industry,  to-day,  depend  abso- 
lutely upon  the  presence  and  application  of  this  energy. 

REFERENCES 

On  the  energy  relationship  of  chemical  change,  consult  W. 
Nernst's  "Theoretical  Chemistry"  (1911),  Books  in  and  IV. 


Section  37 
LIGHT  WAVES  AND   LINES   OF  ELECTRICAL  FORCE 

The  Present  Status  of  the  "ALiher" — Experiment  shows 
that  light  has  many  of  the  properties  of  some  sort  of 
wave  motion.  Since  it  is  difficult  to  imagine  a  wave 
without  thinking  of  some  substance  hi  which  it  is  a  wave, 
physicists  have  been  accustomed  up  to  recent  years  to 
assume  the  existence  of  an  all-pervading  aether,  the 
undulations  of  which  constitute  light  and  other  similar 
disturbances.  Of  late,  however,  certain  very  important 
experimental  and  theoretical  results  —  clustering  around 
what  is  called  the  " principle  of  relativity" — have  thrown 
a  great  deal  of  doubt  upon  the  existence  of  the  aether, 
so  that  it  is  now  advisable  to  conceive  of  a  light  wave  in  a 
somewhat  different  way. 

The  Nature  of  Electrical  Force  Lines.  —  Nearly  every- 
one is  familiar  with  the  fact  that  the  state  of  affairs  in 
the  space  around  a  body  which  is  charged  with  elec- 
tricity can  be  represented  by  what  are  called  "lines  of 
electrical  force."  These  lines  show  in  a  symbolic  fashion 
how  another  charged  body  placed  in  the  space  in  question 
would  tend  to  move,  or,  in  other  words,  what  forces  would 
act  upon  it.  It  is  probable  that  these  lines  of  force  have 

[146] 


Sec.  37]  *  'KINKS"   IN   FORCE   LINES 

some  counterpart  in  reality,  and  recent  developments- 
make  it  not  improbable  that  electrons  and  other  charged 
particles  of  atomic  or  sub-atomic  size  are  actually  centers 
of  radial  "  tubes  of  electrical  force." 

"Kinfe"  in  Electrical  Force  Lines.  —  It  can  be  shown 
that  lines  of  electrical  force,  if  they  exist,  must  possess 
"  inertia,"  that  is,  that  they  must  offer  resistance  to 
changes  in  their  state  of  motion  or  rest.  In  other  words 
they  act  as  regards  motion  very  much  like  a  stiff  rope  or 
wire  attached  to  the  electron  or  other  charged  particle. 
Hence  if  the  electron  is  suddenly  set  into  motion  or  if, 
when  in  motion,  it  is  suddenly  brought  to  rest,  its  lines 
of  force  will  not  accommodate  themselves  to  this  change 
in  motion  immediately,  but  will  tend  to  remain  at  rest  or 
to  keep  on  moving  as  they  did  before  anything  had  hap- 
pened to  the  electron.  This  means  that  every  time  an 
electron  is  slowed  down  or  is  speeded  up,  and  every  time 
the  direction  of  its  motion  is  altered,  kink*  or  curves  will 
be  formed  in  its  lines  of  electrical  force.  The  principle 
of  the  formation  of  these  kinks  is  essentially  the  same  as 
that  of  the  production  of  waves  in  a  rope,  one  end  of 
which  is  shaken.  Just  as  the  waves  in  the  rope  move 
away  from  their  source  at  a  definite  speed,  so  the  kinks 
in  the  electrical  force  lines  travel  away  from  the  electron 
or  other  charges,  with  the  speed  of  light.  The  formation 
of  such  "kinks"  is  illustrated  in  Figure  23. 

If  the  charge  which  is  under  consideration  vibrates 
continuously  back  and  forth,  a  series  of  kinks  will  obvi- 
ously be  formed  in  its  force-lines,  and  these  will  con- 
stitute waves.  As  a  matter  of  fact  light  and  many  other 
forms  of  radiation  are  probably  made  up  of  a  series  of 
just  such  kinks  or  curves. 

Electrons  probably  do  not  vibrate  or  oscillate  as  much 
as  we  were  once  inclined  to  believe.  It  seems  more 

[147] 


LIGHT  WAVES 


[Sec.  37 


likely,  in  the  light  of  recent  developments,  that  they 
move  in  "jerks,"  dropping  suddenly  from  one  position 
to  another  without  an  ensuing  reverse  motion.  If  this 


Fig.  23 

TO    SHOW    HOW    RADIATION    IS    PRODUCED    BY    STOPPING 
THE   MOTION   OF  AN   ELECTRICAL   PARTICLE 

The  diagram  at  the  left  represents  a  charge  of  electricity  with  its  radiat- 
ing lines  of  forces.  We  will  suppose  this  charge  with  its  lines  to  be  mov- 
ing uniformly  in  the  direction  indicated  by  the  arrow  in  the  diagram  at  the 
right.  When  the  charge  is  suddenly  brought  to  rest  the  "lines"  have  a 
tendency  to  continue  in  motion,  and  do  so  until,  so  to  speak,  the  news  of 
the  stopping  of  the  charge  has  reached  them.  This  "news"  travels  out- 
wards from  the  charge  with  the  velocity  of  light,  along  with  the  "kinks "  in 
the  force-lines  which  result  from  the  discrepancy  between  the  actual  and 
the  "expected"  position  of  the  charge.  These  kinks  contain  electro- 
magnetic energy  and  constitute  light  and  other  forms  of  electro-magnetic 
radiation.  Such  radiation  is  produced  whenever  any  change  whatsoever 
occurs  in  the  state  of  uniform  motion  of  an  electrical  charge. 


is  true  it  means  that  the  ordinary  laws  of  motion  do 
not  apply  without  modification  to  the  motion  of  electrons. 
This  question  is  further  discussed  in  Section  54. 

REFERENCES 

On  the  electrical  theory  of  light,  consult  Righi's  "Modern  The- 
ory of  Physical  Phenomena"  (1904),  Chapter  II. 

The  theory  of  "kinked"  force-lines  above  discussed  was  first 

[148] 


Sec.  38]  ZEEMAN   EFFECT 

developed  by  J.  J.  Thomson  and  one  of  his  simplest  accounts  of 
it  will  be  found  in  his  "Electricity  and  Matter"  (1904),  Chapters 
I-in  inclusive. 

See  also  W.  C.  D.  Whetham's  "The  Theory  of  Experimental 
Electricity"  (1912),  Chapter  IX. 

Section  38 
THE  ZEEMAN  EFFECT 

The  rapidity  with  which  any  vibrating  body  moves 
back  and  forth  in  its  path  depends  upon  the  magnitude  of 
the  forces  which  are  acting  upon  it.  Magnetic  forces  are 
known  to  act  upon  moving  electrically  charged  bodies  and 
hence  we  should  expect  that  if  there  are  electrons  vibrat- 
ing or  moving  in  any  way  within  sources  of  light,  the 
application  of  a  magnet  to  such  a  light  source  would  alter 
the  rate  of  vibration  of  these  electrons.  If  this  occurs, 
the  vibration  time,  and  hence  the  " wave-length"  of  the 
light,  must  also  be  changed.  From  the  mathematical 
theory  of  electricity  it  is  possible  to  calculate  the  exact 
change  which  should  occur,  assuming  a  vibrating  particle 
of  known  charge  and  mass.  Conversely,  if  we  know  the 
change  hi  the  character  of  the  light  we  can  estimate  the 
charge  and  mass  of  the  electrical  particle  which  is  emit- 
ting the  light. 

A  very  large  number  of  observations  have  been  made 
on  the  effect  produced  by  magnetic  forces  upon  the  light 
given  off  by  many  different  substances  in  the  glowing 
state.  All  of  these  observations  show  that,  certainly  in 
the  majority  of  cases,  the  vibrating,  or  otherwise  moving, 
particle  is  an  electron. 

The  alteration  of  wave-length  of  the  light  emitted  by 
a  glowing  body  under  the  influence  of  magnetic  forces  is 
known  to  physicists  as  the  "Zeeman  effect,"  after  its 
discoverer  Paul  Zeeman.  (See  Figure  24.) 

[149] 


LIGHT   PRODUCTION 


[Sec.  39 


It  has  been  shown  by  J.  Stark  that  the  mode  of  vibra- 
tion of  the  electrons  within  an  atom  can  also  be  modified 
by  the  application  of  a  strong  electrical  field. 

A  B 


Fig.  24 
THE   ZEEMAN   EFFECT 

The  two  lines  A  and  B  in  1  are  svpposed  to  be  two  lines  in  the  spectrum 
of  a  luminous  element,  such  as  hydrogen  or  mercury  vapor.  When  a 
powerful  magnet  is  applied  to  the  glowing  element  these  spectral  lines 
break  up  into  "triplets"  or  complex  groups  of  triplets,  as  shown  in  2. 
This  is  called  the  Zeeman  effect,  and  in  its  simplest  form  is  readily  ex- 
plained by  the  electron  theory. 


REFERENCES 

See  Sir  Oliver  Lodge's  " Electrons"  (1907),  Chapter  XI;  E.  C.  C. 
Baly's  "  Spectroscopy "  (1905),  Chapter  XIV,  and  Norman  Camp- 
bell's "Modern  Electrical  Theory,"  second  edition  (1913),  pp. 
146-152. 

Zeeman's  own  account  will  be  found  in  his  "Researches  in 
Magneto-optics"  (1913). 


Section  39 
THE  CONDITIONS  UNDER  WHICH  LIGHT  IS  PRODUCED 

Temperature  Radiation;  .  The  Spectral  "Distribution 
Curve."  -There  are  various  special  conditions  under 
which  material  bodies  emit  light.  The  one  which  is  most 
familiar  is  that  of  high  temperature. 

We  have  already  stated  that  the  electrons  in  bodies 
take  part  in  the  heat  vibrations  along  with  the  atoms  and 

[150] 


Sec.  39]  TEMPERATURE  RADIATION 

molecules,  and  if  this  is  true  they  must  constantly  give 
off  electrical  waves.  The  length  of  these  waves  must 
obviously  depend  upon  the  rapidity  of  the  vibrations,  and 
this,  in  turn,  increases  with  the  temperature.  Hence, 
as  the  temperature  of  a  body  containing  electrons  is 
raised,  the  preponderating  length  of  the  waves  which 
it  gives  out  should  become  less.  This  is  exactly  what 
occurs  in  nature. 

When  a  body  is  being  heated  the  first  perceptible  radia- 
tions which  it  emits  are  "heat  waves."  When  the  body 
becomes  "red  hot"  it  is  giving  off  the  longest  of  the  light 
waves,  and  "white  heat,"  which  every  one  recognizes 
to  be  hotter  than  "red  heat,"  is  only  possible  when  green 
and  blue  light  have  been  added  to  the  red.  The  wave- 
lengths of  the  blue  and  green  are  much  shorter  than  that 
of  red  light.  With  still  further  increases  in  temperature, 
the  radiation  begins  to  include  still  shorter  waves  which 
make  up  the  so-called  "ultra-violet"  light. 

On  account  of  the  relationship  which  holds  between 
the  temperature  of  a  hot  substance  and  the  color  of  the 
light  which  it  emits  it  is  possible  to  get  an  idea  of  this  tem- 
perature by  means  of  observations  upon  the  color  of  the 
body.  The  so-called  "optical  pyrometer"  is  an  instru- 
ment based  upon  this  principle,  which  does  for  very  hot 
bodies  what  an  ordinary  thermometer  does  for  cooler 
ones. 

As  we  have  seen  in  Section  23,  all  of  the  molecules,  and 
hence  all  of  the  electrons,  in  a  body  are  not  moving  at 
the  same  velocity  even  if  the  temperature  is  throughout 
what  we  call  "uniform."  Some  are  moving  faster  and 
others  slower,  according  to  the  "law  of  distribution  of 
molecular  speeds,"  as  explained  in  the  Section  referred 
to.  Owing  to  this  fact  we  should  not  expect  the  light  from 
a  body  at  a  given  temperature  to  be  all  of  the  same  wave- 

[151] 


LIGHT  PRODUCTION  [Sec.  39 

length.  Supposing  that  the  particles  whose  vibrations 
are  responsible  for  the  light  are  electrons,  it  is  possible 
to  calculate  the  wave-length  of  the  light  corresponding  to 
the  " average  molecular  energy"  which  is  characteris- 
tic of  a  given  temperature.  Since  there  are  more  elec- 
trons moving  at  a  velocity  corresponding  to  approximately 
this  energy  than  at  any  other  (approximate)  velocity,  we 
should  expect  most  of  the  light  to  be  of  (approximately) 
the  calculated  wave-length.  Measurements  verify  this 
expectation  and  also  the  idea  that  the  electron  is  the  ac- 
tual source  of  the  radiation.  The  form  of  this  "  curve 
of  distribution"  of  energy  in  the  spectrum  of  a  solid  at 
various  temperatures  is  shown  in  Figure  25. 

But  there  are  electrons  moving  at  speeds  both  higher 
and  lower  than  this  "average  speed,"  and  hence  there 
should  be  light  to  correspond,  although  such  light  should 
be  less  intense,  the  further  its  wave-length  departs  from 
that  belonging  to  the  "average  speed."  Observation 
validates  this  conclusion  also. 

However,  the  so-called  law  of  the  distribution  of  light 
intensities  along  the  spectrum  for  any  given  tempera- 
ture is  not  exactly  what  should  be  expected  from  the 
molecular  theory,  in  its  ordinary  form,  and  the  efforts  of 
physicists  to  explain  its  deviation  from  the  expected  form 
have  finally  culminated  in  the  modern  "quantum" 
theory  of  light  which  is  discussed  in  another  place  (see 
Section  54). 

The  theory  of  temperature  radiation  is  based  pri- 
marily upon  the  conception  of  a  "black  body"  which  is 
a  body  absorbing  all  radiation  impinging  upon  it  and 
showing  a  minimum  of  selectivity  in  its  emission  of 
radiation. 

The  Emission  of  Light  by  Gases.  —  Gases  as  well  as 
solids  give  off  light  under  the  right  conditions.  It  is  only 

[152] 


Sec.  39] 


DISTRIBUTION   CURVES 


1650° 


Fig.  25 

CURVES  SHOWING  THE  RELATIVE  INTENSITIES  OF  RADIA- 
TION OF  DIFFERENT  WAVE  LENGTHS  EMITTED  BY  SOLID 
BODIES  AT  VARIOUS  TEMPERATURES 

The  numbers  along  the  horizontal  scale  represent  the  wave-length  of 
the  radiation  (light  or  heat)  in  thousandths  of  a  millimeter.  The  vertical 
scale  represents  the  relative  intensity  of  the  radiation.  It  will  be  observed 
that  the  curves  for  the  higher  temperatures  (given  in  degrees  Centigrade) 
have  their  maxima  at  points  corresponding  with  shorter  waves  than  those 
characteristic  of  the  lower  temperatures.  The  meaning  of  this  fact  is 
explained  in  the  text.  These  curves  are  those  of  the  so-called  "black 
body  radiation." 

[163] 


LIGHT   PRODUCTION  [Sec.  39 

when  a  substance  is  in  the  gaseous  state  that  its  charac- 
teristic "spectrum"  can  be  obtained  distinctly. 

It  has  not  generally  been  considered  possible  to  cause 
a  gas  to  glow  simply  by  heating  it.  The  reason  for  this 
is  to  be  found  in  the  fact  that  under  ordinary  conditions 
gases  contain  very  few  free  electrons  or  ions,  and  that  even 
high  temperatures  will  not  produce  a  sufficient  number 
of  these  to  make  the  gas  luminescent. 

In  order  to  accomplish  this  end,  it  is  necessary  either 
to  send  an  electrical  current  through  the  gas  or  to  permit 
chemical  action  to  take  place  within  it.  We  have  seen 
in  Section  27  that  both  of  these  conditions  favor  ioniza- 
tion.  Ions  can  be  formed  in  a  gas  only  when  electrons  are 
" knocked  out"  of  the  atoms  which  make  up  the  gas,  and 
it  is  the  change  in  the  "mode  of  motion"  of  the  electrons 
which  occurs  either  when  they  are  being  ejected  from  or 
when  they  return  to  the  atoms  which  gives  rise  to  the  glow 
that  accompanies  (say)  the  electrical  discharge  through 
the  so-called  "  vacuum  tube."  The  light  of  the  familiar 
" mercury  vapor  arc"  depends  upon  the  same  principle. 

"Line  Spectra"  -  The  light  which  is  given  off  by 
glowing  gases  differs  from  that  emitted  by  solids  hi 
respect  to  its  "  distribution "  along  the  spectrum.  Prac- 
tically all  of  the  energy  in  the  former  case  is  concentrated 
about  certain  definite  wave-lengths,  the  positions  of  which 
are  not  only  constant  for  a  given  gas,  but  are  different 
for  different  gases.  The  "spectrum"  of  a  glowing  gas, 
then,  shows  a  series  of  "lines"  of  colored  light  in  place 
of  the  continuous  rainbow  band  which  is  produced  when 
light  from  a  white  hot  metal  is  passed  through  a  prism. 
This  special  nature  of  the  spectra  of  substances  in  the 
gaseous  condition  must  be  attributed  to  the  fact  that  the 
electrons  in  such  substances  have  much  less  freedom  of 
movement  than  have  those  within  a  metal,  so  that  they 

[154] 


Sect.  39]  LINE   SPECTRA 

can  only  execute  vibrations  of  a  few  definite  frequencies, 
as  determined  by  the  structure  of  the  atoms  and  mole- 
cules of  the  substance. 

The  study  of  the  line  spectra  of  the  various  elements 
hi  gaseous  form  has  shown  that  in  any  single  element 
the  lines  can  be  arranged  into  "series,"  such  that  the 
positions  of  the  individual  lines  hi  these  series  —  that 
is,  the  wave-lengths  of  the  lights  composing  them  —  can 
be  calculated  from  relatively  simple  mathematical  for- 
mulae. The  exact  nature  of  these  formulae  differs  from 
element  to  element,  although  it  retains  important  points 
of  identity  throughout.  It  has  for  some  time  been  recog- 
nized, on  the  basis  of  the  Zeeman  effect  (which  we  have 
discussed  in  Section  38),  that  the  partial  cause  of  this 
identity  lies  in  the  fact  that,  in  all  of  the  cases  consid- 
ered, the  light  is  emitted  by  electrons.  Only  recently, 
however  (see  Section  53),  have  physicists  been  able  to 
suggest  a  way  in  which  the  relatively  simple  electronic 
structure  attributed  to  such  atoms  as  that  of  hydrogen 
could  be  consistent  with  the  very  complex  nature  of  their 
line  spectra. 

REFERENCES 

A  complete  but  elementary  discussion  of  the  various  forms  of 
electro-magnetic  radiation  is  given  in  S.  P.  Thompson's  "Radia- 
tion" (1898). 

A  more  modern  treatment  is  the  excellent  one  by  N.  Campbell 
in  his  "Modern  Electrical  Theory,"  second  edition  (1913),  Chap- 
ters IX  and  X. 

Section  40 
THE  GAMUT  OF  ELECTRICAL  WAVES 

When  a  beam  of  white  light,  such  as  ordinary  sunlight, 
is  passed  through  a  glass  prism  it  is  broken  up  into  the 
so-called  prismatic  colors,  which  are  arranged  in  the  order 

[155] 


GAMUT  OF  WAVES  [Sect.  40 

of  their  wave-length.  The  shortest  waves  are  repre- 
sented by  the  violet  light,  and  the  longest  by  the  red. 
The  wave-length  of  the  latter  is  about  twice  that  of  the 
former. 

The  arrangement  of  the  colors  in  the  spectrum  produced 
by  a  prism  is  not  such  that  the  distance  of  each  color  from 
the  end  of  the  spectrum  is  proportional  to  its  wave-length. 
This  relationship  does  hold,  however,  in  what  is  called  a 
"normal"  spectrum.  If  a  spectrum  of  this  latter  sort 
about  a  yard  long  could  be  extended  so  as  to  include  all 
known  electro-magnetic  radiations  (among  them  the 
Hertzian  waves)  it  would  become  over  five  million  miles 
in  length. 

Of  those  waves  which  are  shorter  than  light,  the  most 
familiar  are  the  so-called  "ultra-violet"  waves,  which 
lie  just  beyond  the  violet  end  of  the  spectrum,  and  the 
"  X  rays,"  which  recent  experiments  indicate  to  be  differ- 
ent from  ordinary  light  chiefly  in  the  possession  of  an 
extremely  short  wave,  or  "kink"  (see  Section  37),  about 
one  thousandth  that  of  ultra-violet  light.  With  the 
"X  rays"  we  have  to  include  the  so-called  "gamma 
rays"  from  radium  (see  Part  I).  The  existence  of  these 
rays  can  be  detected  by  their  chemical  effects,  for  example 
by  their  power  to  produce  pictures  or  shadows  on  a  photo- 
graphic plate. 

The  waves  which  are  longer  than  light  comprise  the 
so-called  "heat  rays,"  which  affect  our  temperature 
sense,  and  the  Hertz  waves,"  which  are  employed  in 
wireless  telegraphy. 

All  of  these  waves  travel  at  the  same  speed  hi  empty 
space,  D/Z.,  at  the  rate  of  186,300  miles  per  second.  The 
length  of  the  shortest  visible  light  wave  is  about  one 
hundred-thousandth  of  an  inch.  The  longest  Hertzian 
waves  measure  over  a  mile  from  crest  to  crest.  On  ac- 

[156] 


Sec.  41]  SELECTIVE   ABSORPTION 

count  of  the  tremendous  speed  at  which  light  travels  — 
the  highest  speed  known  to  science,  and  perhaps  the 
highest  possible  speed  —  the  rapidity  of  vibration,  or 
the  " frequency"  of  light  as  it  passes  through  a  fixed 
point,  is  extremely  great.  About  eight  hundred  trillion 
waves  of  violet  light  would  pass  through  such  a  point 
in  a  second.  The  extreme  brevity  of  the  interval  of  time 
required  for  the  passage  of  a  single  wave  of  this  sort  — 
or  for  the  completion  of  a  single  oscillation  of  the  generat- 
ing electron  —  may  perhaps  be  realized  better  when  it  is 
said  that  one  eight-hundred-trillionth  of  a  second  is  a 
vastly  smaller  part  of  a  second  than  a  second  is  of  the 
whole  of  historic  time  (i.e.,  one  two-hundred-and-fifty- 
billionth). 

REFERENCES 

A  very  popular  account  of  the  properties  of  light  is  given  in 
" Light,  Visible  and  Invisible"  (1910),  by  Silvanus  P.  Thompson. 


Section  41 

COLOR  AND  THE  ABSORPTION  AND  REFLECTION 
OF  LIGHT 

The  "Selective  Absorption"  of  Light.  — When  light 
passes  through  a  semi-transparent  body  it  always  be- 
comes less  intense,  on  account  of  the  absorption  which 
takes  place.  If  the  original  light  is  colorless  we  generally 
find  that  it  is  more  or  less  colored  when  it  comes  out 
of  the  body.  This  is  due  to  the  fact  that  white  light 
is  a  mixture  of  lights  of  many  different  wave-lengths, 
and  that  these  lights  are  not  all  absorbed  hi  equal 
proportions. 

The  reason  for  this  inequality  of  absorption  is  to  be 
found  in  the  general  explanation  of  the  absorption  of 

[157] 


COLOR  [Sec.  41 

light  which  is  given  in  Part  I.  A  body  absorbs  only  such 
light  as  can  produce  response  in  its  electrons.  If  the 
internal  forces  (see  Section  8)  of  the  molecules  of  a  given 
body  permit  the  electrons  which  they  contain  to  respond, 
to  an  appreciable  extent,  only  to  red  light,  a  beam  of 
white  light  passing  through  such  a  body  will  be  colored 
blue-green,  because  the  red  light  is  removed  by  absorp- 
tion and  its  complementary  —  blue-green  —  is  thus  left 
unbalanced.  Generally  a  body  will  strongly  absorb  light 
of  several  quite  different  wave-lengths. 

The  Sensations  of  Color.  —  The  basis  of  the  sensations 
of  color  is  not  to  be  looked  for  in  the  nature  of  light,  so 
much  as  in  the  nature  of  the  effects  which  light  produces 
in  the  retina  of  the  eye  and  in  the  nerves  which  are  con- 
nected thereto.  Physics  as  such  offers  no  explanation  of 
the  fact  that  light  of  one  wave-length  gives  us  a  sensa- 
tion quality  almost  wholly  different  from  that  produced  by 
light  of  another  wave-length.  Neither  does  it  account  for 
the  fact  that  a  mixture  of  lights  of  many  different  wave- 
lengths gives  white.  These  are  problems  in  physiology 
although  their  solution  is,  of  course,  closely  connected 
with  the  physics  of  light. 

How  Color  is  Produced  by  Reflection.  —  When  light  is 
colored  by  reflection  from  the  surface  of  a  body,  as  for 
example,  from  a  piece  of  green  paper,  we  must  suppose 
the  process  to  be  in  reality  an  absorption  phenomenon, 
since  most  of  the  light  undoubtedly  penetrates  the  body 
to  a  certain  extent  before  it  is  reflected.  It  thus  passes 
through  a  portion  of  the  body  in  two  directions,  and  dur- 
ing this  passage  is  subjected  to  the  ordinary  conditions  of 
absorption. 

Bodies  differ  in  their  power  to  reflect  light,  primarily 
on  account  of  the  fact  that  the  electrons  which  they 
contain  are  not  similarly  conditioned  by  their  surround- 

[  158  ] 


Sec.  42]  REFRACTION 

ings.    The  best  reflectors  will  in  general  be  the  metals, 
since  these  contain  free  electrons  in  large  numbers. 


REFERENCES 

For  a  semi-popular  discussion  of  color  and  absorption,  refer 
to  "Light"  by  R.  C.  Maclaurin,  Chapters  II  and  in.  See  also 
Franklin  and  MacNutt's  "Light  and  Sound"  (1909),  especially 
Chapter  X. 

A  very  recent  and  comprehensive  exposition  of  color  problems 
is  that  of  M.  Luckiesh,  "Color  and  Its  Applications  (1915). 


Section  42 
THE  REFRACTION  OF  LIGHT 

How  Columns  of  Light  are  Bent.  —  Light  may  be 
thought  of  as  moving  in  columns  or  "pencils"  the  fronts 
of  which  are  perpendicular  to  the  direction  of  motion  of 
the  light.  When  the  front  of  a  column  strikes  the  surface 
of  a  transparent  body  the  edge  which  meets  the  surface 
first  must  be  retarded  in  its  motion  if,  as  is  generally  the 
case,  light  moves  slower  in  the  body  than  it  does  in  empty 
space.  Hence,  the  edge  in  question  falls  behind  the  other 
parts  of  the  front  and  the  plane  of  the  front  is  rotated. 
Since  in  general  the  column  will  move  in  a  direction  at 
right  angles  to  the  plane  of  the  front,  the  light  is  bent  at 
the  surface  of  the  body.  If  the  column  of  light  strikes  the 
surface  at  right  angles  there  will  be  no  bending  because 
all  parts  of  the  front  will  hit  the  surface  at  the  same  time. 
The  more  acute  the  angle  of  impact,  the  greater  will  be 
the  bending. 

This  bending  of  a  light  column  or  pencil  when  it  passes 
through  a  surface  obliquely  is  called  refraction.  As  is 
well  known,  refraction  lies  at  the  basis  of  the  effects  pro- 

[159] 


REFRACTION   AND   DISPERSION       [Sec.  42 

duced  by  all  lenses  and  prisms.  To  explain  the  details 
of  the  process  is  beyond  the  scope  of  this  book,  since 
they  are  complicated  and  do  not  depend  in  any  especially 
significant  way  upon  the  modern  theory  of  matter,  but  it 
should  be  noted  that  refraction  depends  solely  on  the 
fact  that  the  velocity  of  light  is  different  inside  and  out- 
side of  the  medium  in  question. 

"Dispersion."  -The  same  substance  refracts  light 
waves  of  different  lengths  to  different  extents.  This 
produces  what  is  known  as  dispersion,  a  process  upon 
which  the  formation  of  spectra  by  prisms  is  based.  It 
has  been  shown  experimentally  that  the  degree  in  which 
a  given  substance  refracts  light  of  any  specified  vibration 
period  is  closely  related  with  the  natural  period  of  vibra- 
tion of  the  molecules  of  the  refracting  substance  itself. 
As  the  vibration  period  of  the  transmitted  light  approaches 
that  of  the  substance,  the  refraction  increases  enormously, 
and  then  changes  suddenly.  It  is  this  difference  in  the 
refractive  power  of  a  single  substance  for  different  wave- 
lengths which  makes  possible  the  " dispersion"  of  color, 
as  in  the  ordinary  prismatic  spectrum. 

"Dielectric  Capacity"  and  the  "Index  of  Refraction"  — 
The  degree  to  which  a  substance  refracts  light  of  speci- 
fied wave-length  is  called  its  index  of  refraction  for  this 
light,  and  it  has  been  shown  that  a  definite  relation- 
ship exists  between  the  dielectric  capacity  of  a  substance 
and  its  index  of  refraction.  (See  Section  33  for  another 
connection  of  dielectric  capacity.)  For  many  solids  and 
liquids  this  relationship  is  a  complicated  one  when  the 
frequency  of  the  light  considered  is  close  to  that  of  the 
molecules  of  the  substance  itself.  However,  in  the  case 
of  gases,  and  if  sufficiently  long  waves  are  used,  for  all 
bodies,  the  refractive  index  is  found  to  be  proportional  to 
the  square-root  of  the  dielectric  capacity. 

[160] 


Sec.  43]  ROWLAND'S  EXPERIMENT 

The  dielectric  capacity  of  a  body  measures  the  ease 
with  which  the  electrical  components  of  its  molecules 
undergo  temporary  separation  (without  decomposition  of 
the  molecule)  through  the  activity  of  outside  electrical 
forces.  In  general,  the  more  the  particles  of  a  substance 
respond  to  the  force  of  a  light  wave  the  greater  is  the 
effect  upon  the  speed  of  the  wave.  Hence  it  is  easy  to 
see  why,  within  the  above-mentioned  limits,  a  substance 
which  has  a  high  dielectric  capacity  should  also  have  in 
general  a  high  index  of  refraction. 

Both  refraction  and  absorption  occur  to  the  greatest 
extent  when  the  frequency  of  the  light  which  is  passing 
through  a  body  most  closely  approximates  the  natural 
frequency  of  vibration  of  the  molecules  of  the  substance, 
since  under  these  conditions  the  response  of  these  mole- 
cules is  the  greatest  possible. 

REFERENCES 

On  refraction  and  reflection,  consult  R.  C.  Maclaurin's  "Light," 
Chapter  VI. 

Section  43 
ROWLAND'S  EXPERIMENT 

The  fact  that  a  magnetic  field  is  produced  by  the  mo- 
tion of  an  electrically  charged  body  was  proven  by  the 
late  Professor  Rowland  of  Johns  Hopkins  University. 

Professor  Rowland's  experiment  was  a  very  simple 
one.  Everybody  is  familiar  with  the  fact  that  the  presence 
of  a  magnetic  force  can  always  be  detected  by  the  deflec- 
tion which  it  causes  in  the  position  of  a  compass  needle. 
Some  strips  of  gold-leaf  were  cemented  upon  a  hard 
rubber  disk,  and  after  they  had  been  charged  with  elec- 
tricity the  disk  was  rotated  very  rapidly.  The  needle  of 

[161] 


ELECTRONS  AND   MAGNETISM        [Sec.  44 

a  delicate  compass  placed  near  by  showed  a  distinct 
deflection,  which  could  only  be  ascribed  to  the  magnetic 
forces  generated  by  the  motion  of  the  charged  strips  of 
gold-leaf. 

The  relation  between  the  direction  of  the  magnetic 
forces  and  that  of  the  moving  electricity  or  electric  current 
is  shown  in  Figure  26. 


Fig.  26 

THE   DIRECTION   OF   THE   MAGNETIC   FORCES   ABOUT  A 
MOVING   ELECTRICAL   CHARGE 

Magnetic  forces  exist  around  every  moving  electrical  charge.  If  the 
charge  is  positive  and  is  moving  in  the  direction  of  the  large  arrow  the 
magnetic  forces  will  possess  the  arrangement  and  direction  indicated  by 
the  black  circle  and  its  small  arrows.  If  the  charge  is  negative  it  must 
move  in  the  opposite  direction  to  produce  the  same  magnetic  effect. 

REFERENCES 

On  the  relation  between  electricity  and  magnetism,  see  Oliver 
Lodge's  "Modem  Views  of  Electricity,"  Chapter  VII. 


Section  44 

THE  DEFLECTION  OF  MOVING  ELECTRONS  BY  A 
MAGNET 

The  principle  which  is  employed  in  the  dynamo  for 
the  generation  of  the  electrical  current  is  illustrated  in 
certain  modern  experiments  and  observations  to  which 
we  have  already  referred  in  several  places. 

[162] 


Sec.  45]         DIA-  AND   PARA-MAGNETISM 

In  Section  25  we  have  mentioned  the  fact  that  the 
so-called  cathode  rays,  which  in  reality  consist  of  very 
rapidly  moving  electrons,  are  capable  of  being  deflected 
by  a  magnet.  Where  the  rays  impinge  upon  the  walls  of 
the  "vacuum  tube"  in  which  they  are  produced  they 
cause  a  bright  spot  of  light.  If  a  strong  magnet  is  brought 
near  the  tube  this  spot  of  light  is  seen  to  move. 

This  deflection  of  the  rapidly  moving  charges  in  the 
cathode  rays  has  been  very  carefully  studied  and  the 
results  embodied  in  what  is  believed  to  be  one  of  the  most 
fundamental  and  exact  of  electromagnetic  laws; —  "The 
Law  of  the  Deflection  of  Moving  Charges  in  a  Magnetic 
Field." 

REFERENCES 

See  J.  J.  Thomson's  "The  Discharge  of  Electricity  Through 
Gases"  (1898),  Chapter  on  the  "Cathode  Rays,"  p.  137;  and 
Oliver  Lodge's  "Electrons"  (1906),  Chapters  VTH-XIX  inclusive. 

Section  45 
ALL  BODIES  ARE  MAGNETIC 

Dia-  and  Para-magnetism.  —  One  ordinarily  thinks  of 
the  vast  majority  of  substances  as  "non-magnetic." 
Iron  and  steel  seem  to  be  marked  out  from  other  bodies 
by  their  possession  of  magnetic  powers.  As  a  matter  of 
fact,  however,  every  known  substance  possesses  magnetic 
properties. 

There  are  two  kinds  of  magnetism,  dia-  and  para- 
magnetism.  When  a  dia-magnetic  body  is  placed  in  the 
field  of  a  strong  magnet  its  long  axis  tends  to  turn  so  as 
to  be  at  right-angles  to  the  direction  of  the  lines  of  mag- 
netic force.  Para-magnetic  bodies,  on  the  other  hand, 
tend  to  place  their  long  axes  parallel  to  their  lines  of 
force. 

[163] 


MAGNETISM  [Sec.  45 

The  fact  that  all  bodies  have  magnetic  properties  in- 
dicates from  the  present  point  of  view  that  they  all  contain 
electrons  in  motion.  Whether  a  particular  body  is  dia-  or 
para-magnetic  probably  depends  upon  the  exact  way  in 
which  the  electrons  are  moving  and  upon  the  conditions 
which  limit  this  motion. 

Permanent  Magnetism.  —  Iron,  cobalt  and  nickel  differ 
from  other  substances  hi  their  power  to  acquire  strong 
permanent  magnetism.  This  property  may  be  explained 
in  the  following  manner.  If  there  are  electrons  rotating 
within  or  about  the  atoms  of  a  substance,  each  atom  will 
behave  like  a  very  small  magnet.  But  in  a  visible  body 
there  would  be  so  many  of  these  minute  magnets  with 
their  positive  and  negative  poles  pointing  "every  which 
way"  that  the  resulting  outside  effect  would  be  prac- 
tically nil. 

In  order  that  such  an  external  effect  should  be  pro- 
duced it  would  be  necessary  for  a  number  of  the  little 
magnets  to  point  hi  the  same  general  direction,  so  that  the 
individual  influences  would  be  added  to,  instead  of  neu- 
tralizing, each  other.  To  bring  the  atoms  into  line  in 
this  way  an  outside  magnetic  force  might  be  applied, 
for  it  is  known  from  experiments  with  a  compass  needle 
that  one  magnet  can  cause  another  to  turn  upon  its  axis. 
It  is  probably  some  such  reaction  of  the  atomic  magnets 
as  this  which  produces  all  of  the  phenomena  of  dia-  and 
para-magnetism.  In  the  majority  of  bodies  the  atoms  do 
not  remain  in  magnetic  line  when  the  outside  force  is 
removed.  But  in  others  they  do  tend  to  remain  in 
line  and  these  latter  are  said  to  possess  permanent 
magnetism. 

If  magnetization  actually  does  involve  a  turning  of  the 
molecules  on  then*  axes,  we  should  anticipate  that  a  cer- 
tain amount  of  the  energy  which  is  used  in  the  process 

[164] 


Sec.  46]  RADIO-ACTIVE   ELEMENTS 

might  be  converted  into  heat,  since  additional  motion  is 
imparted  to  the  molecules.  This  effect  can  actually  be 
observed  in  the  case  of  the  more  magnetic  substances, 
and  is  at  the  basis  of  a  troublesome  loss  of  energy  in 
dynamo  work  technically  called  "  hysteresis." 

REFERENCES 

An  exhaustive  discussion  of  "Magnetism"  by  Shelf ord  Bidwell 
will  be  found  in  the  eleventh  edition  of  the  "  Encyclopaedia  Bri- 
tannica."  For  a  simple  and  briefer  account  see  Norman  Campbell, 
loc.  cit.,  Chapter  V. 

Section  46 
RADIO-ACTIVE  SUBSTANCES 

The  Discovery  of  the  Radio- Elements.  —  The  first  sub- 
stance found  to  be  radio-active  was  uranium.  The  dis- 
covery was  made  by  Becquerel  in  1896.  M.  and  Mme. 
Curie,  however,  soon  found  that  the  ore  from  which 
uranium  was  extracted  was  four  times  as  radio-active  as 
pure  uranium  itself,  and  this  observation  led  to  the  dis- 
covery of  radium,  a  new  chemical  element  having  a 
radio-activity  over  a  million  times  greater  than  that  of 
uranium.  Later  on  the  same  investigators  laid  bare  fur- 
ther and  even  more  powerful  radio-active  bodies:  polo- 
nium and  actinium.  Another  element,  thorium,  already 
known  to  chemists,  was  found  by  Schmidt  to  be  radio- 
active to  about  the  same  extent  as  uranium. 

"Disintegration  Series"  —  A  complete  list  of  the  radio- 
active elements  is  given  in  Table  II  of  Section  5.  Most 
of  the  substances  named  in  this  table  are  what  may  be 
called  "  radio-active  products,"  that  is,  they  are  bodies 
which  are  produced  in  the  course  of  radio-activity.  Ra- 
dium, for  example,  apparently  breaks  up  into  helium  and 
"  niton,"  both  of  which  are  gases.  The  emanation 

[165] 


RADIO-ACTIVITY  [Sec.  46 

(niton)  decomposes,  in  turn,  forming  a  solid  substance, 
radium  A,  which  is  soon  converted  by  a  similar  change 
into  radium  B,  and  so  on.  It  is  supposed  that  the  final 
product  of  this  series  of  changes  is  chemically  identical 
with  lead. 

Similar  " disintegration  series"  of  substances  are  de- 
rived from  uranium,  thorium,  and  actinium,  as  shown  in 
the  table  (II,  Section  5).  A  series  may  " branch"  at 
certain  points.  It  is  now  believed  that  both  radium  and 
actinium  are  ultimately  derived  from  uranium,  by  such 
a  process. 

Besides  these  special  radio-active  bodies  others  have 
now  been  shown  to  possess  slight  radio-active  powers. 
Many  investigators  believe  that  all  substances  are  some- 
what radio-active,  but  it  is  not  yet  entirely  certain  whether 
the  faint  activity  which  exists,  is  an  intrinsic  property  of 
the  substances  or  whether  it  is  to  be  ascribed  to  the 
presence  within  them  of  small  traces  of  the  radio-active 
bodies  proper. 

The  Law  of  Decay  of  Radio-Adice  Substances.  —  As 
stated  in  Part  I,  the  rate  of  decay  of  a  radio-element, 
so  far  as  yet  found,  is  independent  of  all  external  condi- 
tions. The  law  of  this  decay  is  that,  for  a  given  element, 
the  same  fraction  of  any  specific  volume  will  always  break 
up  in  the  same  period  of  time.  Thus,  if  we  should  start 
with  an  ounce  of  radium  A,  it  would  be  hah*  gone  at  the 
end  of  three  minutes.  In  three  minutes  more  one-half 
of  the  remainder  —  or  one-quarter  of  the  original  amount, 
in  addition  —  would  have  broken  up,  and  so  on.  It  is 
clear  that  a  law  of  change  of  this  sort  would  theoretically 
never  lead  to  the  total  disintegration  of  any  given  quantity 
of  the  substance,  however  small.  This  is  why  the  "life " 
of  a  radio-element  is  stated  in  terms  of  the  time  required 
for  one-half  of  a  given  amount  of  it  to  decay. 

[166] 


Sec.  46]  LAWS   OF  RADIO-ACTIVITY 

These  " half-times"  vary  from  about  twenty-six  bil- 
lion years  in  the  case  of  thorium  to  only  one  ten-billionth 
of  a  second  in  the  case  of  radium  C'.  The  intensity  of 
the  radiation  from  any  radio-active  substance  is  naturally 
inversely  proportional  to  the  time  required  for  a  quantity 
of  it  to  decompose.  It  has  been  shown  by  Geiger  and 
Nuttall  that  the  shorter  the  period  of  lif  e  of  a  radio-element 
the  faster  its  alpha  particles  move.  There  is  a  similar 
relationship  for  the  beta  particles. 

The  Position  of  the  Radio-Elements  in  the  Periodic  Table. 
—  The  position  of  the  principal  radio-active  elements  in 
the  periodic  table  is  especially  worthy  of  notice.  It  will 
be  seen  (Section  6)  that  not  only  are  they  among  the 
heaviest  of  the  elements,  but  that  all  of  the  heaviest  ele- 
ments are  radio-active.  Apparently  the  relative  insta- 
bility of  the  radio-elements  is  a  corollary  of  complexity  of 
internal  structure,  and  is  analogous  to  the  unstable  char- 
acter of  the  higher  chemical  complexes,  such  as  the 
organic  compounds  of  which  living  bodies  are  made  up. 
However,  mere  weight  is  not  the  only  factor  involved, 
since  uranium,  the  heaviest  element  known,  and  the 
parent  of  the  majority  of  the  radio-active  substances,  is 
one  of  the  least  active  of  them  all.  It  is  estimated  that 
the  time  of  decay  of  uranium  is  long  compared  with  the 
age  of  many  of  the  minerals  hi  which  it  is  found.  If  it 
were  not  for  this  fact,  and  the  similarly  low  activity  of 
thorium,  all  signs  of  radio-activity  would  have  disap- 
peared from  the  earth  long  ago.  It  is  probable  that  other 
series  of  radio-elements,  perhaps  of  even  greater  atomic 
weight,  have  existed  in  the  past,  but  have  left  no  recog- 
nizable traces  behind  them. 

REFERENCES 

E.  Rutherford:  " Radio- Active  Substances  and  their  Radiations" 
(1913),  (Standard  treatise). 

[167] 


METHODS   IN   RADIO-ACTIVITY        [Sec.  47 

F.  Soddy:  "The  Interpretation  of  Radium"  (1912)  (popular 
lectures),  and  "The  Chemistry  of  the  Radio-Elements"  (1915). 

R.  K.  Duncan:  "The  New  Knowledge"  (1905),  Parts  4  and  5, 
pp.  87-193  (popular). 

C.  W.  Raffety:  "An  Introduction  to  the  Science  of  Radio- 
Activity"  (1909). 

A.  T.  Cameron:   "Radio-Chemistry,"  (1910). 


Section  47 
HOW  THE  RAYS  FROM  RADIUM  ARE  STUDIED 

We  have  seen  in  Part  I  and  in  Section  25  that  moving 
electrically  charged  bodies  can  be  turned  from  their 
straight-line  paths,  or  deflected,  by  the  action  of  a  mag- 
net. Hence  it  is  possible  to  get  a  first  indication  as  to 
which,  if  any,  of  the  rays  from  radium  are  electrical 
particles,  and  which  are  electrical  waves,  by  placing  a 
magnet  across  their  paths.  The  first  type  of  rays  will  be 
deflected  while  the  second  will  not.  The  extent  to  which 
they  are  deflected,  when  combined  with  certain  other 
measurements  which  have  been  mentioned  hi  Section  25, 
shows  the  speed  at  which  they  are  travelling,  as  well  as 
the  sign  of  their  charge  and  the  ratio  of  this  charge  to 
their  mass. 

When  an  experiment  of  this  sort  is  tried  upon  a  bundle 
of  rays  from  radium,  certain  of  the  rays  are  found  to  be 
uninfluenced  and  to  move  on  in  a  straight  line  as  before. 
Another  set  is  deflected  slightly,  while  a  third  suffers 
very  marked  change  in  path.  The  last  two  are  diverted 
in  opposite  directions,  which  proves  then-  charges  to  be 
positive  and  negative  respectively.  The  first  type  of 
rays  are  the  "  gamma  rays,"  the  second  the  "  alpha 
rays,"  and  the  third  the  "beta  rays."  The  methods 
which  are  used  in  measuring  the  speeds,  charges  and 

[168] 


Sec.  48]      NATURE  OF  ALPHA  RAYS 

masses  of  the  alpha  and  beta  particles  are  similar  to  those 
described  in  Section  25  for  the  "  cathode  rays." 

The  fact  that  the  beta  particles  can  penetrate  even  thick 
pieces  of  solid  matter  is  proven  by  simply  directing  a 
pencil  of  beta  rays  towards  a  plate  composed  of  the  solid 
substance  in  question,  and  noticing  that  the  rays  appear 
in  a  somewhat  diffused  and  attenuated  state  on  the  other 
side  of  the  plate. 

As  already  mentioned,  C.  T.  R.  Wilson  has  devised  a 
method  by  means  of  which  it  is  possible  to  photograph 
the  paths  of  single  alpha  particles  in  their  motion  through 
a  gas. 

REFERENCES 

The  methods  employed  in  the  study  of  radio-activity  are  popu- 
larly discussed  by  R.  K.  Duncan  in  his  "New  Knowledge"  (1908), 
Part  IV. 

Section  48 

HOW  RUTHERFORD  PROVED  THE  ALPHA  RAYS  TO 
BE  HELIUM   ATOMS 

The  fact  that  the  alpha  rays  are  atoms  of  helium  was 
proved  in  a  very  striking  and  simple  experiment  by  Ernest 
Rutherford.  Helium  is  a  gas,  and  it  is  a  well-known  fact 
that  every  gas  when  subjected  to  the  action  of  an  electri- 
cal discharge  gives  off  light  of  a  peculiar  color,  which  can 
be  split  up  by  a  glass  prism  into  the  so-called  " spectrum" 
of  the  gaseous  substance  (see  Section  39).  Rutherford 
very  carefully  removed  all  traces  of  helium  from  a  large 
glass  tube  and  then  placed  within  this  tube  a  smaller  one 
of  very  thin  walls  containing  gaseous  helium.  After  per- 
mitting the  apparatus  to  stand  for  several  days  he  passed 
an  electrical  discharge  through  the  larger  tube,  and  ex- 
amined the  light  to  see  if  the  spectrum  of  helium  was 
present.  He  found  it  to  be  absent,  showing  that  ordinary 

[169] 


NATURE   OF   GAMMA  RAYS  [Sec.  49 

helium  could  not  pass  through  the  walls  of  the  smaller 
tube. 

He  now  replaced  this  latter  tube  by  an  identical  one 
containing  not  helium  but  radium  emanation,  which  is 
constantly  throwing  off  alpha  rays.  Since  the  walls  of  the 
tube  were  thin  the  great  speed  and  energy  of  the  particles 
in  the  rays  enabled  them  to  penetrate  the  walls  and  enter 
the  atmosphere  of  the  larger  vessel. 

After  the  apparatus  had  stood  under  these  conditions 
for  a  period  of  time  Rutherford  again  passed  an  electrical 
discharge  through  it,  and  found  that  it  distinctly  showed 
the  spectrum  of  helium.  This  seems  to  be  conclusive 
proof  that  the  alpha  ray  particles  are  actually  atoms 
of  helium. 

The  mass  or  weight  of  the  alpha  particles  has  also  been 
measured  and  has  been  shown  to  be  of  a  magnitude  in 
harmony  with  this  conclusion. 

REFERENCES 

See  Rutherford's  "  Radio-Active  Substances  and  their  Radia- 
tion" (1913),  pp.  137-140  and  Chapter  XVII. 


Section  49 
THE  NATURE  OF  THE  GAMMA  RAYS 

We  have  seen  in  Section  37  that  electrical  waves  can  be 
regarded  as  kinks  hi  lines  of  electrical  force  which  are 
produced  whenever  the  velocity  of  motion  of  an  electri- 
cally charged  particle  is  altered.  Now  we  know  that  the 
beta  particles  bear  electrical  charges,  and  that  they  are 
suddenly  emitted  from  the  atoms  of  radio-active  sub- 
stances at  a  tremendous  speed.  In  accordance  with  the 
theory,  then,  they  ought  to  give  rise  to  electrical  waves 
of  very  high  frequency,  that  is,  the  kinks  which  are  pro- 

[170] 


Sec.  49]  SECONDARY  RAYS 

duced  in  their  lines  of  force  should  be  exceedingly  sharp. 
This  conclusion  seems  to  harmonize  with  what  we  know 
about  the  gamma  rays,  which  are  apparently  made  up  of 
the  electrical  waves  in  question. 

When  the  beta  particles  are  stopped  in  their  headlong 
flight  by  striking  the  atoms  of  some  other  substance  we 
should  expect  further  gamma  (or  X)  rays  to  be  produced, 
because  any  change  hi  the  velocity  of  one  of  these  par- 
ticles should  give  rise  to  an  electrical  wave,  whether  the 
change  be  of  the  nature  of  an  acceleration  or  a  retarda- 
tion. It  has  been  found  by  experiment  that  such  rays 
are  actually  produced  under  the  circumstances  specified. 
They  belong  to  the  class  of  " secondary  rays"  hi  which, 
also,  must  be  included  the  further  beta  radiation  which  is 
set  up  by  the  gamma  rays  themselves  when  they  are 
absorbed  by  material  bodies. 

Another  type  of  secondary  rays  produced  under  en- 
tirely analogous  conditions  and  probably  of  the  same  gen- 
eral nature  are  the  well-known  "X  rays."  Certain  very 
recent  experiments  which  prove  beyond  a  doubt  that 
the  latter,  and  probably  also  the  gamma  rays,  are  not 
moving  material  or  electrical  particles,  in  the  usual  sense, 
will  be  discussed  in  Section  55.  In  that  place,  also,  will 
be  considered  the  explanation  of  the  great  power  of 
gamma  and  X  rays  to  penetrate  bodies  opaque  to  ordi- 
nary light. 

REFERENCES 

Concerning  the  gamma  rays,  see  R.  K.  Duncan's  "The  New 
Knowledge"  (1908),  pp.  109-112.  Also  Norman  Campbell's  "Mod- 
ern Electrical  Theory,"  second  edition  (1913),  pp.  273 /. 


[171] 


INTRA-ATOMIC  ENERGY  [Sec.  50 

Section  50 
THE  ENERGY  OF  THE  ATOM 

The  enormous  energy  of  the  inner  constitution  of  the 
atom  is  probably  very  closely  connected  with  the  great 
stability  of  atoms  in  general.  Even  the  atoms  of  radium 
are  only  relatively  unstable,  since  if  we  consider  any 
single  radium  atom  it  has  what  we  might  call  an  "ex- 
pectation of  lif e "  of  several  thousand  years.  The  great 
stability  of  the  atom  is  due  to  the  fact  that  the  forces 
which  hold  the  parts  of  the  atom  together  are  very  great, 
and  the  magnitude  of  these  forces  is  closely  associated 
with  that  of  the  intra-atomic  energy. 

We  have  several  times  spoken  of  the  forces  of  chemical 
affinity  and  cohesion  as  residual  in  character,  as  repre- 
senting the  attractions  which  are,  so  to  speak,  "left  over" 
after  the  parts  of  the  atom  have  been  cemented  together. 
If  chemical  energy  (for  example)  is  a  residue  of  the  intra- 
atomic  energies  we  should  expect  it  to  be  relatively  much 
smaller  than  these  energies,  just  as  the  stability  of  the 
molecule  is  relatively  much  less  than  that  of  the  atom. 
We  have  become  accustomed  to  the  quantities  of  energy 
liberated  in  chemical  changes  and  have  taken  them  as 
standards,  so  that  when  we  come  to  consider  the  primary 
energies  of  the  atoms  these  seem  unbelievably  great. 

REFERENCES 

Concerning  intra-atomic  energies  read  R.  K.  Duncan  "The 
New  Knowledge,"  Part  5,  Chapter  HI.  The  energy  liberated  in 
radio-activity  is  discussed  by  J.  J.  Thomson  in  his  "Electricity 
and  Matter"  (1904),  pp.  152 /. 


[172] 


Sec.  61-52]    RADIO-ACTIVITY   OF  POTASSIUM 

Section  51 
THE  RADIO-ACTIVITY  OF  POTASSIUM 

As  nearly  everyone  knows,  the  element  potassium  is 
a  constituent  of  ordinary  caustic  potash.  Yet  this  common 
element  has  been  shown  by  Norman  Campbell  to  be 
definitely  radio-active.  The  rays  which  it  emits  appear 
to  be  principally  of  the  "beta"  type,  the  intensity  of  the 
rays  from  potassium  being  about  one  one-thousandth 
that  of  the  beta  rays  from  uranium.  All  of  the  potassium 
salts  are  radio-active,  and  thus  far  no  evidence  has  been 
adduced  to  show  that  this  activity  is  due  to  impurities. 

Other  of  the  so-called  alkali  metals,  for  example  rubid- 
ium, have  been  shown  to  possess  slight  radio-activity. 

REFERENCES 

See  Norman  Campbell:  "The  Radio-Activity  of  Potassium" 
in  the  Proceedings  of  the  Cambridge  (Eng.)  Philosophical  Society 
for  1908,  Vol.  14,  pp.  657-567.  Also  Campbell's  "  Modern  Elec- 
trical Theory,"  second  edition  (1913),  pp.  187-188. 

Section  52 
INORGANIC  EVOLUTION 

We  have  seen  in  Section  39  that  all  of  the  elements 
possess  characteristic  " spectra"  which  consist  of  series  of 
lines.  For  most  elements  these  series  are  quite  complex 
and  the  positions  of  the  separate  lines  in  the  spectrum, 
that  is,  the  wave-lengths  of  the  lights  which  compose 
them,  do  not  change.  However,  the  exact  number  of 
lines  which  are  present  depends  to  a  certain  extent 
upon  the  conditions  under  which  the  element  is  made 
luminous.  The  spectrum  from  a  flame  has  fewer  lines 
than  that  from  an  electric  arc. 

Now  astronomic  observations  have  shown  that  in  the 
[173] 


INORGANIC   EVOLUTION          [Sec.  52-53 

light  from  many  stars  the  spectra  of  certain  elements 
are  curiously  incomplete.  In  general,  the  hotter  a  star 
is  the  more  incomplete  the  spectra  of  its  elements  appear. 
If  we  suppose  that  the  different  sets  of  lines  hi  the  spec- 
trum of  the  element  are  produced  by  the  vibrations  of 
different  electrons,  or  systems  of  electrons,  within  the 
atom,  it  is  natural  to  infer  that  the  simplification  of  the 
spectra  in  the  hottest  stars  stands  for  an  actual  breaking 
up  of  the  atoms  in  these  stars. 

But  in  addition  to  this  it  has  been  shown,  by  Sir  Nor- 
man Lockyer  on  the  basis  of  spectroscopic  evidence,  that 
hi  the  very  hottest  stars  are  to  be  found  only  the  simplest 
elements,  such  as  hydrogen,  helium,  etc.,  along  with 
partly  formed  elements  of  higher  atomic  weight.  The 
cooler  a  star  is  the  more  elements  it  contains,  and  the 
higher  are  the  atomic  weights  of  these  elements. 

These  facts  suggest  that  the  elements  are  undergoing 
an  actual  evolution  in  certain  of  the  heavenly  bodies,  an 
evolution  which  depends  primarily  upon  the  fact  that 
these  bodies  are  passing  through  a  process  of  cooling. 
The  lightest  and  simplest  elements  are  formed  first, 
and  after  them  the  heavier  and  more  complex  ones. 
Among  the  latter  are  the  radio-active  substances. 

REFERENCES 

Sir  Norman  Lockyer's  own  account  will  be  found  in  his  "Inor- 
ganic Evolution,  as  Studied  by  Spectrum  Analysis."  A  simpler 
presentation  of  the  facts  is  given  by  R.  K.  Duncan  in  Part  VI  of 
"The  New  Knowledge"  (1908). 

Section  53 
THEORIES   OF  THE  STRUCTURE  OF  THE  ATOM 

Thomsons  Theory. — Up  to  very  recent  times  the  most 
promising  conception  of  atomic  structure  was  that  elabo- 
rated by  Sir  J.  J.  Thomson  in  a  highly  mathematical 

[174] 


Sec.  63]  THOMSONIAN   ATOM 

paper  published  in  the  English  Philosophical  Magazine,  in 
1904.  Although  it  is  practically  certain  that  the  theory, 
as  originally  stated,  is  not  accurately  true,  it  must  never- 
theless be  admitted  that  no  other  view  has  appeared 
which  gives  us  an  equal  feeling  of  insight  into  the  mystery 
of  the  chemical  elements. 

On  the  basis  of  the  known  laws  of  electrical  attraction, 
Thomson  calculated  the  constitution  of  the  series  of  hy- 
pothetical atoms  which  would  be  generated  by  the  suc- 
cessive addition  of  electrons  to  a  large  sphere  of  positive 
electricity  always  of  sufficient  charge  to  just  neutralize 
the  electrons.  For  simplicity,  he  assumed  the  electrons 
to  be  concentrated  in  a  single  plane.  He  was  able  to 
show  that  the  electrons  would  arrange  themselves  into 
rings  and  that  with  an  increase  in  the  total  number  there 
would  be  a  periodically  recurring  tendency  for  fresh  rings 
to  be  formed,  in  addition  to  those  already  present.  How- 
ever, the  latter,  also,  would  be  obliged,  from  time  to  time, 
to  increase  their  electronic  contents  in  order  to  maintain 
the  stability  of  the  system. 

With  the  formation  and  development  of  each  new  ring 
the  properties  of  the  atoms  might  be  expected  to  repeat 
to  a  limited  extent,  those  which  were  passed  through  in 
the  development  of  the  previously  formed  ring.  The 
presence  of  the  latter  —  owing  to  the  general  modifica- 
tion of  the  forces  of  the  system  which  it  would  involve 
-would  preclude  a*n  exact  repetition  of  properties.  It 
is  clear  that  a  very  close  analogy  exists  between  this 
theoretical  series  of  atoms  of  Thomson's  and  the  actual 
system  of  the  elements,  as  revealed  in  the  periodic  table. 
The  first  member  in  each  " period"  in  the  table  (see  Sec- 
tion 6)  may  be  supposed  to  coincide  with  the  formation 
of  a  new  ring,  which  would  contain,  in  the  given  element, 
only  one  electron. 

[175] 


STRUCTURE   OF  ATOM  [Sec.  63 

As  stated  in  Part  I,  the  recent  "nucleus  theory"  of 
the  structure  of  the  atom  supposes  that  the  positive  elec- 
tricity, instead  of  forming  a  large  sphere,  coextensive 
with  the  general  volume  of  the  atom,  is  concentrated 
in  a  very  minute  central  region.  However,  we  still  hear 
of  concentric  rings,  or  shells,  of  electrons  surrounding 
this  nucleus,  and  it  is  probable  that,  in  a  general  way,  the 
arrangement  of  the  electrons  resembles  that  in  the 
Thomsonian  atom. 

The  Nucleus  Theory.  —  The  " nucleus  theory"  was 
proposed  by  Rutherford  to  explain  the  manner  in  which 
the  alpha  rays  from  radio-active  substances  are  scat- 
tered as  a  result  of  their  passage  through  material  bodies. 
This  scattering  may  be  supposed  to  be  caused  by  the  ac- 
tion of  the  intrinsic  forces  of  the  atoms  of  the  body  upon 
the  charged  particles  which  make  up  the  rays.  The 
degree  of  scattering  is  measured  by  the  angle  made  by 
the  path  of  the  ray  as  it  leaves  the  thin  sheet  of  substance, 
through  which  it  has  passed,  and  its  original  line  of 
travel.  Now  experiment  shows  that  even  when  the 
average  degree  of  scattering  is  relatively  low,  a  small 
fraction  of  the  rays  are  turned  through  a  very  large  angle, 
often  so  that  their  motion  is  actually  reversed  in  direction. 

Since  the  original  speed  of  the  alpha  particles  and  their 
charge  are  known,  it  is  possible  to  calculate  what  con- 
ditions would  be  necessary  to  cause  a  deflection  of  this 
magnitude.  In  the  case  of  the  element  gold,  such  calcu- 
lations show  that  the  positive  electricity  of  the  atom 
must  be  concentrated  on  a  sphere  about  one  trillionth  of 
an  inch  in  diameter,  which  is  only  one  ten-thousandth 
part  of  the  diameter  of  the  atom  itself. 

It  is  estimated  that  in  passing  through  hydrogen  gas, 
some  of  the  alpha  ray  particles  come  within  one  twenty- 
five-trillionth  of  an  inch  of  the  centers  of  the  positive 

[176] 


Sec.  53]         NUCLEUS  THEORY   OF  ATOM 

nuclei.  Since  this  is  less  than  the  diameter  of  an  electron 
it  seems  probable  that  the  bare  positive  nucleus  is  smaller 
than  are  the  negative  particles  hi  the  atom.  In  Section 
25  we  have  seen  that  on  the  assumption  that  the  electron 
is  made  of  pure  negative  electricity,  it  is  possible  from  a 
knowledge  of  its  mass  and  charge  to  calculate  its  size. 
Now,  the  facts  indicate  that  the  mass  (or  weight)  of  the 
positive  component  of  the  atom  is  enormously  greater 
than  that  of  the  electron,  so  much  so  that  it  is  practically 
equivalent  to  the  total  weight  of  the  atom.  On  this  basis, 
calculation  shows  that  the  nucleus  of  the  hydrogen  atom 
—  if  its  mass  is  wholly  electrical  —  must  have  a  diameter 
of  about  one  ten-quadrillionth  of  an  inch,  or  one  eighteen- 
hundredth  that  of  the  electron.  This  result  appears  to 
be  in  harmony  with  that  reached  by  the  study  of  the  alpha- 
ray  scattering. 

The  supposition  that  the  positive  components  of  the 
nucleus  of  most  atoms  have  electrons  closely  bound  up 
with  them  is  necessitated  by  the  facts  of  radio-activity. 
The  tremendous  speed  with  which  the  electrons  of  the 
beta  rays  are  sent  off,  demands  original  intra-atomic 
forces  which  could  only  result  from  an  exceedingly  close 
packing  together  of  the  atomic  parts  which  are  involved. 
Moreover,  there  seems  little  room  for  doubt  that  there 
are  at  least  two  classes  of  electrons  in  the  atom,  (1)  those 
which  directly  determine  its  physical  or  chemical  proper- 
ties, and  a  fraction  of  which  can  be  separated  from  the 
atom  and  replaced  again  without  great  difficulty,  and  (2) 
those  which  leave  the  atom  only  during  a  radio-active 
change,  and  the  loss  of  which  means  an  apparently 
irrevocable  alteration  in  the  fundamental  nature  of  the 
element. 

It  seems  probable,  however,  that  electron  groups  of 
varying  degrees  of  superficiality,  so  to  speak,  exist  within 

[177] 


STRUCTURE   OF   ATOM  [Sec.  53 

complex  atoms.  These  may  be  thought  of  as  correspond- 
ing with  the  successive  rings  of  the  Thomsonian  atom. 
The  most  superficial  system  of  all  is  that  of  the  so-called 
valency  electrons,  which  are  probably  responsible  for  the 
more  obvious  chemical  properties  of  the  substance  and, 
as  Stark  believes,  for  its  band  spectra.  Still  deeper 
electron  layers  give  rise  to  the  recently  discovered  X  ray 
spectra  (see  Section  55). 

The  Number  of  Electrons  in  the  Atom.  —  Numerous 
attempts  have  been  made  to  calculate  the  number  of 
electrons  in  the  atoms  of  the  various  elements.  This 
can  be  done  on  the  basis  of  the  degree  hi  which  rays  of 
several  sorts  are  scattered  in  passing  through  sheets  of  the 
elements  in  question.  The  more  electrons  there  are- 
that  is,  obstacles  for  the  rays  to  encounter  —  the  more 
scattering  will  occur.  Results  obtained  by  a  number  of 
different  methods  indicated  that  the  number  of  electrons 
is  approximately  equal  to  one-half  the  atomic  weight. 
But  it  is  clear  that  if,  as  in  the  case  of  hydrogen,  one 
unit  of  positive  charge  is  always  associated  with  one  unit 
of  weight,  the  number  of  electrons  in  the  atom  would 
have  to  be  accurately  equal  to  the  atomic  weight  in  order 
to  balance  the  charge  of  the  nucleus.  We  are  led,  there- 
fore, to  suppose  that  there  are  as  many  electrons  bound 
up  in  the  nucleus  as  there  are  in  the  body  of  the  atom. 
The  number  of  these  latter,  " external"  electrons  is  con- 
trolled by  the  magnitude  of  the  unbalanced  charge  of  the 
nucleus,  which  is  represented  by  the  "atomic  number" 
(vide  infra).  As  a  matter  of  fact,  as  an  examination 
of  Table  I,  Section  5,  will  show,  the  nuclei  of  the 
heavier  elements  must  contain  more  electrons  than  the 
body  of  the  atom. 

Isotopism.  —  Radio-activity,    as    we    have    stated,    is 
indubitably  an  affair  of  the  nucleus,  and  it  appears  to 

[178] 


Sec.  53]    ISOTOPES   AND   ATOMIC    NUMBERS 

depend  upon  the  ejection  either  of  a  doubly,  positively, 
charged  helium  atom  —  alpha  particle  —  or  a  singly, 
negatively,  charged  electron  —  beta  particle.  It  is  ob- 
vious that  if  the  nucleus  loses  one  alpha  particle  and 
then  two  electrons  its  resultant  charge  will  be  the  same 
as  before  these  three  radio-active  changes  had  occurred. 
Consequently,  the  number  and  arrangement  of  its  exter- 
nal electrons  will  be  the  same  as  before  the  change,  and 
the  two  elements,  although  differing  in  atomic  weight 
by  four  units,  will  have  identical  chemical  and  physical 
properties.  As  shown  in  Table  II,  Section  5,  many  such  so- 
called  isotopes  have  been  demonstrated  among  the  radio- 
active elements.  A  mixture  of  isotopes  acts  like  —  and 
indeed  is  —  a  chemically  pure  substance,  and  there  is  no 
chemical  reaction  which  can  be  used  to  separate  its 
components.  However,  these  components  can  easily  be 
distinguished  from  each  other  by  radio-active  tests. 
Chemical  tests  distinguish  between  only  ten  kmds  of 
radio-elements,  whereas  radio-active  tests  show  thirty- 
four  or  more. 

Atomic  Numbers.  —  The  expulsion  of  an  alpha  particle 
from  an  atom  causes  the  corresponding  element  to  move 
two  places  from  right  to  left  in  the  periodic  table  (see 
Section  6);  the  loss  of  a  beta  particle  reverses  this  move- 
ment one  place.  It  appears,  then,  that  the  position  of  an 
element  in  the  table  depends  not  upon  its  atomic  weight, 
directly,  but  upon  the  resultant  positive  charge  of  its 
nucleus,  —  which  is  called  its  atomic  number,  —  and  the 
successive  places  in  the  table  correspond  with  unit  dif- 
ferences in  this  charge.  Consequently,  in  so  far  as  the 
table  is  regarded  as  a  chemical  schema,  its  principle  should 
be  restated  as  follows:  "All  of  the  chemical  properties 
of  the  elements  are  periodic  functions  of  their  atomic 
numbers  (instead  of  weights)."  Chemical  analysis,  it 
\[179] 


STRUCTURE   OF  ATOM  [Sec.  63 

would  appear,  is  really  an  analysis  of  matter  only  into 
different  types,  not  into  different  elements. 

The  recent  discovery  of  a  means  of  measuring  the  wave- 
lengths of  X  rays  has  shown  that  all  of  the  elements, 
under  the  right  conditions,  give  off  X  rays,  the  lengths 
of  which  are  characteristic  of  the  element  in  question. 
It  has  been  found  possible  to  calculate  these  wave-lengths 
by  means  of  a  very  simple  formula  involving  the  atomic 
number  or,  vice  versa,  to  deduce  the  atomic  number  from 
the  wave-length.  Measurements  of  this  sort  by  Moseley 
indicate  that  the  atomic  number  of  gold  is  79,  and  that 
from  aluminium  to  gold,  in  the  periodic  table,  only  three 
possible  elements  are  missing.  The  atomic  number  of 
the  heaviest  element,  uranium,  appears  to  be  92,  although 
there  is  some  disagreement  concerning  this  point.  If  this 
is  correct  and  if  uranium  is  the  heaviest  element  which 
can  exist,  then  it  means  that  only  92  different  chemical 
elements  are  possible. 

The  Hydrogen  Atom.  —  In  some  respects  hydrogen 
occupies  a  unique  position  hi  the  system  of  the  elements. 
The  alpha  ray  particle,  which  is  a  helium  atom  with  a 
double  positive  charge,  may  be  thought  of  as  consisting 
of  the  helium  nucleus  stripped  of  its  external  electrons. 
The  ordinary  hydrogen  ion,  bearing  one  positive  charge,  is 
probably  the  bare  nucleus  of  the  hydrogen  atom.  How- 
ever, the  alpha  particle  contains  four  units  of  weight  to 
two  of  charge,  while  the  hydrogen  ion  has  one  unit  of 
weight  to  one  of  charge.  On  the  basis  of  the  argument 
previously  presented  it  would  appear  that  while  the  helium 
nucleus  has  bound  up  with  it  two  electrons,  the  hydrogen 
nucleus,  or  ion,  consists  of  pure  positive  electricity,  and 
is,  hi  fact,  a  positive  electron. 

On  the  supposition  that  the  uncharged  hydrogen  atom 
is  thus  made  up  of  two  electrical  particles,  one  positive 

[180] 


Sec.  53]  THE   HYDROGEN   ATOM 

and  the  other  negative,  the  Dutch  physicist,  Bohr,  has 
shown  that  by  use  of  the  known  electro-magnetic  laws 
and  the  new  "quantum"  theory  of  light  (see  Section  54), 
some  very  remarkable  conclusions  can  be  reached.  For 
a  long  time  it  has  been  known  that  the  position  of  the 
" lines"  in  the  hydrogen  spectrum  can  be  calculated  with 
amazing  exactness  by  means  of  a  certain  mathematical 
formula  (named  after  its  discoverer,  Balmer)  which  had 
been  arrived  at  empirically  by  a  method  of  trial  and  error. 
Up  to  the  recent  work  of  Bohr,  however,  it  seemed  im- 
possible to  derive  this  formula  by  means  of  the  laws  of 
simple  mechanics  and  electricity  from  any  conceivable 
hypothesis  about  the  structure  of  an  atom,  especially 
from  a  simple  one.  But  by  introducing  the  assumptions 
of  the  new  theory  of  light,  Bohr  has  shown  that  the  for- 
mula in  question  follows  very  simply  from  the  conception 
of  the  hydrogen  atom  above  described.  This  result  can 
hardly  be  considered  other  than  epoch-making  in  the 
history  of  atomic  and  optical  theory. 

Bohr's  theory  disposes  of  a  difficulty  which  has  bothered 
physicists  for  some  time,  u/z.,  the  problem  as  to  why  the 
electrons  within  the  atom  continue  to  rotate  about  the 
center  of  attraction,  instead  of  rapidly  falling  into  it. 
Centrifugal  force  would  keep  them  out  as  long  as  they 
maintained  their  speeds,  but  the  ordinary  laws  of  radia- 
tion demand  that  these  speeds  should  constantly  dimin- 
ish, owing  to  the  continuous  emission  of  energy  in  the 
form  of  electrical  waves.  However,  the  quantum  theory 
necessitates  that  such  emission  should  occur  only  in 
discontinuous  units  of  fixed  magnitude.  Consequently, 
it  would  be  impossible  for  the  electron  to  drop  gradually 
into  the  atom;  it  would  be  obliged  to  fall  in  steps  or  jerks. 
Each  jerk  would  generate  a  characteristic  wave,  corre- 
sponding with  a  definite  line  in  the  spectrum  of  the  ele- 

[181] 


THE    QUANTUM   THEORY  [Sec.  64 

ment  —  it  being  consistent  with  the  facts  to  suppose  that 
line  spectra  are  formed  only  when  electrons  are  separated 
from  or  returned  to  a  definite  place  in  the  atom.  When 
the  electron  has  completed  any  drop  and  radiated  the 
corresponding  energy  it  remains  in  equilibrium,  probably 
rotating  about  the  nucleus,  but  no  longer  radiating. 

It  is  difficult  to  handle  mathematically  the  problems 
connected  with  the  exact  structure  of  atoms  more  com- 
plex than  that  of  hydrogen,  but  it  seems  probable  that 
these  atoms  are  made  up  of  larger  numbers  of  positive 
particles,  and  electrons,  rotating  in  more  complex  ways. 
It  is  apparent  that  there  is  a  close  analogy  between  the 
structure  and  internal  processes  of  atoms,  as  conceived 
in  modern  theory,  and  those  of  astronomic  systems. 
This  is  why  the  type  of  atom  above  considered  is  some- 
times characterized  as  the  "Saturnian  atom." 

REFERENCES 

J.  J.  Thomson's  theory  of  atomic  structure  is  well  summarized 
in  popular  form  by  R.  K.  Duncan  in  "The  New  Knowledge "  (1908), 
Part  5,  Chapter  II.  Thomson's  own  account  appears  in  popular 
form  in  his  "Electricity  and  Matter"  (1904),  Chapter  V. 

See  also  Norman  Campbell's  "Modern  Electrical  Theory," 
Second  edition  (1913),  Chapter  XIII.  Bohr's  articles  are  in  the 
Philosophical  Magazine  for  July,  September,  and  November,  1913, 
Vol.  26,  pp.  1,  476  and  857.  J.  J.  Thomson's  latest  theory  will 
be  found  in  the  same  journal  for  October,  1913,  p.  792. 

On  isotopes  see  Frederick  Soddy's  "The  Chemistry  of  the 
Radio-Elements"  (1915),  Part  II.  On  atomic  numbers  and  the 
Bohr  atom:  W.  H.  and  W.  L.  Bragg's  "  X  Rays  and  Crystal  Struc- 
ture" (1915),  pp.  77  to  87. 

Section  54 
THE   QUANTUM  THEORY  OF  RADIANT  ENERGY 

The  Nature  of  the  Theory.  —  When  light  travels  from 
one  part  of  space  to  another  there  is  a  motion  of  a  certain 

[182] 


Sec.  54]  LIGHT  ATOMS 

amount  of  "radiant  energy"  through  the  intervening  dis- 
tance, and  at  any  time  during  the  motion  this  energy 
must  be  localized  in  a  definite  region  of  space.  The  ques- 
tion as  to  whether  radiant  energy  is  or  is  not  atomic  may 
consequently  be  asked  in  the  following  way.  Is  light 
energy  travelling  in  free  space  spread  out  uniformly  and 
continuously  in  that  space,  or  is  it  concentrated  hi  a 
limited  number  of  relatively  small  and  clearly  defined 
regions,  the  amount  and  intensity  of  the  energy  in  each 
of  these  regions  being  invariable  for  a  given  kind  of 
light? 

The  first  alternative  is  the  one  which  had  been  accepted 
up  to  quite  recent  times.  It  assumed  that  radiant  energy 
can  be  emitted  from  bodies  continuously,  in  any  amount 
and  at  any  intensity,  that  it  spreads  out  uniformly  from 
its  source  like  a  perfect  non-molecular  fluid,  becoming 
steadily  weaker  the  further  it  goes  from  the  source,  ac- 
cording to  the  well-known  "  law  of  inverse  squares." 
The  second  alternative,  which  corresponds  to  the  modern 
doctrine  of  light  "  quanta,"  denies  all  of  these  assump- 
tions. It  states  that  light  is  not  radiated  from  bodies 
continuously,  but  instead  in  sudden  outbursts,  each  of 
which  is  of  definite  magnitude  and  intensity  determined 
only  by  its  wave-length  or  frequency.  It  would  be  im- 
possible for  a  body  to  radiate  a  fraction  of  one  of  these 
units,  so  that  quantities  of  radiation  which  are  not  inte- 
gral multiples  of  the  units  in  question  cannot  exist. 

Moreover,  the  intensity  of  one  of  these  light  atoms,  or 
quanta,  does  not  decrease  with  its  distance  from  the 
source,  and  consequently  it  does  not  spread  out  as  it 
travels.  The  reason  that  light  seems  to  fall  off  in  intensity 
with  distance  lies  in  the  fact  that  the  number  of  light 
atoms  to  be  encountered  in  a  given  region  of  space  natu- 
rally becomes  less  the  farther  that  region  is  from  the  emit- 

[183] 


THE    QUANTUM   THEORY  [Sec.  54 

ting  body.  This  conception  of  the  structure  of  a  beam  of 
light  means  that  all  optical  images  are  really  built  up  on 
the  same  principle  as  the  ordinary  "  half  -tone"  engrav- 
ing, that  is,  they  are  made  of  minute  dottings  or  stipplings 
far  too  small  to  be  detected  by  the  eye.  (However,  the 
sensitiveness  of  the  retina  is  so  great  that  a  visual  sensa- 
tion can  be  produced  by  relatively  few  quanta  of  the 
right  kind  of  light.) 

Such  a  striking  alteration  as  this  in  the  theory  of  light 
cannot  be  without  strong  grounds.  In  discussing  these, 
no  attempt  will  be  made  to  follow  the  order  of  their  his- 
torical appearance  in  connection  with  the  problem. 

The  Photo-Electric  Effect.  —  We  have  seen  in  Chapter 
V,  that  many  solid  bodies,  especially  metals,  under  the 
influence  of  high  temperatures,  give  off  electrons.  It 
has  been  found  that  they  also  can  be  made  to  emit  elec- 
trons at  ordinary  temperatures  if  their  surfaces  are 
exposed  to  the  action  of  ultra-violet  light  or  X  rays.  If 
we  are  not  to  assume  that  the  metal  becomes  radio-active 
under  the  action  of  the  light,  we  must  suppose  that  the 
energy  of  motion  of  these  emitted  electrons  comes  from 
the  light  itself.  The  very  curious  fact  now  appears  that 
this  energy  —  that  is,  the  highest  speed  with  which  any 
of  the  electrons  travel  —  is  independent  of  the  intensity 
of  the  light  which  shines  upon  the  surface.  If  the  light 
is  weak,  relatively  only  a  few  electrons  are  given  off, 
but  those  which  do  appear  have  the  same  velocity  which 
is  characteristic  of  the  effect  with  lights  of  higher  intensity. 
These  results  seem  to  be  compatible  only  with  some  such 
notion  of  the  atomic  structure  of  light  as  we  have  just 
outlined. 

Measurements  upon  the  speeds  of  the  electrons  in  the 
photo-electric  effect  —  as  the  phenomenon  is  called  — 
prove  that  their  energy  of  motion,  although  independent 

[184] 


Sec.  54]       THE   PHOTO-ELECTRIC   EFFECT 

of  the  intensity,  is  closely  proportional  to  the  "frequency" 
of  the  rays  employed,  that  is,  the  shorter  the  "wave- 
length "  of  the  radiation  the  faster  the  emitted  electrons 
move.  This  connection  between  "frequency"  and  energy 
is  one  of  the  fundamental  principles  of  the  quantum 
theory;  it  resolves  itself  ultimately  into  the  statement 
that  the  energy  of  any  light  quantum  is  directly  propor- 
tional to  its  frequency.  The  higher  the  frequency  the 
higher  the  energy,  but  for  a  given  frequency  the  energy 
is  fixed  and  invariable. 

If  this  principle  is  valid,  it  is  clear  that  it  should  be  im- 
possible to  generate  quanta  of  high  frequency  from  those 
of  low  frequency,  since  this  would  contradict  the  law  of 
the  conservation  of  energy.  In  accordance  with  this,  it 
has  been  shown  experimentally  that  when  one  kind  of 
radiation  is  changed  into  another  —  as,  for  example, 
" fluoresence "  and  "phosphorescence"  —  the  alteration 
in  wave-lengths  is,  in  most  cases,  from  high  to  low 
frequency.  This  empirical  principle  is  known  as  Stores' 
law.  It  holds  for  the  transformation  of  X  rays,  as  well 
as  for  those  of  ordinary  light. 

Such  transformations  as  these  must  always  be  accom- 
plished by  permitting  the  light  to  fall  upon  some  material 
body,  and  there  is  practically  no  doubt  that  the  active 
factors  in  the  change  are  the  electrons  which  are  bound 
up  in  the  atoms  of  the  body.  Experiment  makes  it  prob- 
able that  all  such  electrons  have  natural  rates  of  vibration, 
which  depend  upon  the  constitution  of  the  atoms  or  mole- 
cules of  which  they  form  parts.  The  photo-electric  effect 
and  its  analogues  in  all  probability  depend  upon  the 
ejection  of  electrons  from  atoms  under  the  influence  of 
the  electrical  forces  in  the  light  ray.  The  quantum  theory, 
however,  makes  it  seem  probable  that  (1)  the  energy  of 
the  light  cannot  be  transferred  to  the  electron  unless  the 

[185] 


THE    QUANTUM   THEORY  [Sec.  64 

frequency  of  the  light  is  at  least  approximately  the  same 
as  that  natural  for  the  electron  itself,  and  (2)  if  the  elec- 
tron takes  up  any  of  the  energy  of  the  light  quantum  it 
must  take  it  all.  In  harmony  with  this  view  it  is  found 
that  so  far  as  present  measurements  permit  us  to  judge, 
the  energy  of  the  electrons  emitted  under  the  action  of 
light  and  X  rays  is  the  same  as  that  of  the  respective  light 
quanta. 

Other  Fads  Underlying  the  Theory.  —  There  are  fur- 
ther important  results  of  the  idea  that  an  electron,  atom, 
or  molecule  can  take  on  or  part  with  only  whole  light 
quanta,  and  only  such  quanta  as  have  approximately  their 
own  natural  frequencies.  For  example,  it  can  be  shown 
theoretically,  upon  certain  reasonable  assumptions  with 
regard  to  the  molecular  conditions  underlying  the  facts 
of  specific  heat,  that  if  the  quantum  theory  is  true  the 
specific  heats  of  all  substances  at  or  near  absolute  zero, 
should  themselves  be  very  close  to  zero.  At  low  tem- 
peratures the  average  energy  of  vibration  of  the  atoms  is 
so  small  that  only  a  few  of  them  are  able  to  retain  whole 
quanta  of  energy,  and  if  they  are  unable  to  retain  whole 
quanta  they  cannot  have  any  energy  at  all.  Consequently, 
at  low  temperatures  the  atoms  of  a  body  lose  their  power 
to  absorb  heat,  and  hence  the  body  suffers  a  radical  de- 
cline in  its  specific  heat.  (See  Section  24.)  The  recent 
experimental  work  of  Nernst  and  his  co-workers  has 
shown  that  such  changes  in  the  specific  heats  of  bodies 
actually  do  occur,  and  that  their  manner  of  occur- 
rence satisfies  expectations  based  upon  the  quantum 
theory. 

However,  probably  the  most  important  consideration  of 
all  —  which  is  presented  at  some  length  in  Chapter  X, 
and  which,  unfortunately,  is  too  involved  to  be  developed 
much  more  completely  here  —  is  that  which  first  led  the 

[186] 


Sec.  54]     HIGH  AND   LOW  TEMPERATURES 

German  physicist,  Max  Planck,  to  propound  the  quantum 
theory.  It  concerns  the  manner  in  which  the  amounts  of 
light  of  different  wave-lengths  emitted  by  glowing  solid 
bodies  at  various  temperatures  are  related  with  the  wave- 
lengths and  temperatures  in  question.  This  relation  is 
represented  in  the  " curve  of  distribution"  of  energy  in 
the  spectrum,  which  has  been  discussed  hi  Section  39. 
For  any  given  temperature  there  is  a  certain  wave-length 
which  has  a  higher  intensity  than  any  other.  Wave- 
lengths greater  or  less  than  this  have  increasingly  lower 
intensities.  Various  only  partially  successful  attempts 
had  been  made  by  several  physicists  to  explain  the  exact 
relations  between  temperature,  wave-length,  and  inten- 
sity in  terms  of  the  electron  theory  of  radiation.  Planck 
found  that  such  an  explanation  could  be  given  if  it  be 
assumed  that  the  radiation  takes  place  discontinuously, 
and  that  each  quantum  of  light  radiated  has  an  energy 
proportional  to  its  frequency,  i.e.,  inversely  proportional 
to  its  wave-length.  The  demonstration  of  the  possibility 
of  such  an  explanation  proved  to  be  of  epoch-making 
importance  in  theoretical  physics. 

Significance  of  the  Quantum  Theory. — The  possible  far- 
reaching  significance  of  these  developments  in  the  theory 
of  radiation  may  perhaps  be  suggested  by  a  few  state- 
ments, the  whole  meaning  of  which,  however,  can  only 
be  appreciated  by  one  closely  acquainted  with  physical 
science  and  its  history.  In  the  first  place,  it  has  been 
shown  conclusively  by  the  English  physicist,  Jeans,  that 
although  the  facts  just  mentioned  as  Planck's  first  basis 
for  the  quantum  theory  demand  an  atomic  view  of  the 
nature  of  radiation,  the  interpretation  given  to  them 
makes  them  wholly  inconsistent  with  the  most  funda- 
mental principles  of  the  science  of  mechanics.  In  other 
words,  it  would  appear  that,  to  a  certain  extent  at  least, 

[187] 


THE   QUANTUM   THEORY  [Sec.  54 

events  in  the  world  of  atoms  and  electrons  do  not  follow 
the  laws  of  ordinary  mechanics. 

Secondly,  it  appears  that  these  events  do  involve  in  a 
very  definite  way  certain  considerations  based  upon  the 
theory  of  probabilities  or  chance.  For  some  time  it  has 
been  known  that  the  so-called  second  law  of  thermody- 
namics, which  states  that  the  amount  of  available  energy 
in  the  universe  tends  constantly  to  decrease,  could  be 
derived  from  a  study  of  the  relative  probabilities  of  given 
configurations  of  the  molecules  composing  any  group  of 
bodies.  Certain  arrangements  of  the  molecules  are  more 
probable  as  the  outcome  of  a  disturbance  than  are  others. 
There  would  always  be  a  tendency  for  the  less  probable 
configurations  to  be  replaced  by  the  more  probable  ones. 
This  tendency  corresponds  with  the  statement  that  the 
" entropy"  of  a  given  group  of  bodies  tends  always  to 
increase;  the  greater  the  entropy  of  the  group  the  less 
available  energy  there  is  in  it,  the  nearer  all  of  energy 
in  the  system  is  to  being  uiiiformly  distributed  heat. 
This  latter  state  of  affairs  is  the  most  " probable"  of  all 
states  of  the  molecular  system,  and  the  " entropy"  of  the 
system  as  a  whole  is  merely  another  expression  for  the 
probability  of  the  particular  configuration  of  molecules 
or  molecular  conditions  existing  within  it. 

In  order  that  considerations  of  this  sort,  involving  the 
doctrine  of  chance,  should  be  applicable  to  a  subject 
matter,  it  is  absolutely  necessary  that  this  subject  matter 
consist  of  discrete  individuals  or  particles,  in  short  that 
it  be  atomic.  Now,  it  has  been  shown  very  decisively 
that  the  conception  of  entropy  and  the  principles  of  thermo- 
dynamics at  large  are  definitely  applicable  to  the  be- 
havior of  radiant  energy,  and  hence  we  must  almost 
inevitably  conclude  that  such  energy  is  atomic  hi  nature. 

It  is  of  course  quite  possible  that  radiant  energy  is 
[188] 


Sec.  66]  NATURE   OF   X  RAYS 

atomic  while  other  forms  of  energy  are  not,  that  there 
will  be  a  limit  to  the  application  of  the  principles  of 
atomism  to  the  physical  universe.  Indeed,  certain  well- 
known  physicists  still  hold  that  even  the  facts  which 
support  the  quantum  theory  of  light  can  be  explained 
without  any  radical  change  in  our  present  doctrines.  The 
exact  outcome  of  this  contention  still  rests  in  the  balance. 

REFERENCES 

A  remarkably  complete,  although  somewhat  mathematical  dis- 
cussion of  the  Quantum  Theory  and  its  grounds  is  given  by  Norman 
Campbell  in  his  "Modern  Electrical  Theory,"  second  edition 
(1913),  Chapter  X.  A  somewhat  simpler,  but  also  very  clear  ac- 
count is  that  by  R.  A.  Millikan,  "Atomic  Theories  of  Radiation," 
in  the  journal  Science  for  January  24,  1913  (Vol.  37,  pp.  119-133). 
For  a  popular  discussion  of  the  growth  of  atomic  theories  in  phys- 
ics see  Sir  Oliver  Lodge's  "Continuity"  (1914). 

Section  55 
X   RAYS  AND  THEIR  MEASUREMENT 

The  Origin  and  Nature  of  X  Rays.  —  X  rays  are  formed 
when  the  cathode  rays,  of  which  we  have  spoken  in 
Section  25,  are  stopped  hi  their  course  by  striking  a  solid 
plate,  commonly  called  the  "  anti-cathode."  Since  the 
electrons  of  the  cathode  rays  are  moving  much  more 
slowly  than  are  those  of  the  beta  rays  (see  Section  49), 
the  " kinks"  (see  Section  37)  of  which  they  are  made  up 
are  not  so  sharp,  but  they  are  nevertheless  very  much 
sharper  than  those  of  ordinary  light. 

As  mentioned  incidentally  hi  Section  53,  it  has  been 
shown  that  a  fraction  of  the  X  rays  given  off  by  the  anti- 
cathode  have  a  wave-length  and  penetrative  power  which 
is  characteristic  of  the  element  of  which  the  anti-cathode 
is  composed.  These  " characteristic  X  rays"  are  of 

[189] 


X  RAYS  [Sec.  55 

shorter  wave-length  the  higher  the  atomic  weight  of  the 
element,  and  are  wholly  independent  of  its  state  of 
chemical  combination.  Most  of  the  elements  give  out 
characteristic  X  rays  of  two  different  wave-lengths,  and 
when  these  are  analyzed  by  an  X  ray  spectrometer,  they 
form  the  "X  ray  spectrum"  of  the  element  in  question. 

Why  X  Rays  Penetrate  "Opaque"  Bodies.  — It  is  the 
extreme  "sharpness,"  or  very  high  frequency,  of  the 
gamma  and  X  rays  which  chiefly  distinguishes  them 
from  ordinary  light,  and  which  gives  them  their  special 
power  to  penetrate  solid  bodies  which  are  opaque  to  light. 
Bodies  absorb  light  only  because  their  atoms  are  able  to 
respond,  or  "  resonate,"  to  the  vibrations  in  the  light 
wave  (see  Section  41).  But  when  these  vibrations  are 
very  rapid  —  or,  what  means  the  same  thing,  when  the 
"kinks"  are  very  short — the  atoms  of  the  substance  can- 
not respond  with  sufficient  quickness,  and  hence  the  light 
is  not  absorbed.  There  is  some  question  as  to  whether 
this  explanation  holds  exactly  on  the  quantum  theory  of 
radiation.  However,  there  is  doubtless  also  a  specific 
relation  between  frequency,  as  such,  and  degree  of  ab- 
sorption, since  certain  bodies  strongly  absorb  X  rays  of 
one  frequency  and  not  of  others. 

The  Corpuscular  Properties  of  X  Rays.  —  Certain  phys- 
icists, among  them  W.  H.  Bragg,  formerly  supported  the 
view  that  the  gamma  and  the  X  rays  are  not  waves  but 
are  moving  particles.  The  reason  for -this  advocacy  lay 
hi  the  fact  that  these  rays,  in  passing  through  matter, 
behave  as  if  their  energy  were  concentrated  in  very  mi- 
nute, moving  regions,  instead  of  being  spread  out  over  a 
continuous  "  wave-front,"  as  light  energy  is  supposed  to 
be  in  the  classical  theory.  However,  as  we  have  seen 
in  Section  54,  latter-day  developments  have  proved  that 
ordinary  light  has  the  same  sort  of  distribution,  so  that 

[190] 


Sec.  55]  PROPERTIES   OF   X  RAYS 

the  evidence  in  question  counts  no  more  against  the  wave- 
theory  of  X  rays  than  it  does  against  that  of  light  in  gen- 
eral. Recent  developments,  briefly  discussed  in  Part  I, 
have  completely  converted  Bragg  and  his  school  to  the 
wave  theory  or  at  least  to  the  general  theory  of  radiation. 
The  Reflection  and  "Diffraction"  of  X  Rays  by  Crystals. 
—  The  reflection  of  X  rays  from  a  crystal  surface  is 
somewhat  different  from  that  of  ordinary  light.  The 
reason  for  this  is  to  be  found  in  the  fact  that  although  the 
atoms  of  the  crystal  are  regularly  arranged,  they  are  still 
so  far  apart  compared  with  the  wave-length  of  the  rays 
that  the  discontinuities  produce  a  sensible  effect.  In 
Chapter  VI  of  Part  I  the  reflection  of  ordinary  light  has 
been  explained  as  due  to  the  generation  of  a  return  wave 
by  the  electrons  of  the  atoms  hi  the  surface  of  a  body 
which  is  struck  by  light.  This  is  what  occurs  also  in  the 
case  of  X  rays,  but  owing  to  their  penetrative  power  and 
relatively  short  wave-length  the  return  waves  from  dif- 
ferent planes  of  atoms  in  the  crystal  do  not  fuse  with 
each  other  harmoniously,  except  hi  certain  favorable 
directions. 

In  other  directions  there  is  interference,  that  is,  the 
waves  from  one  layer  of  atoms  oppose  those  from  another 
layer  and  wipe  them  out.  In  fact,  the  X  rays  are  so  short 
that  the  atoms  of  the  crystal  form  for  them  a  "diffrac- 
tion grating,"  similar  in  action  to  the  mechanically  ruled 
gratings  used  to  bring  about  the  interference  of  ordinary 
light  waves. 

Now,  the  direction  in  which  interference  does  not  take 
place  in  the  case  of  X  ray  reflection  depends  upon  the 
wave-length  of  the  rays.  Consequently,  if  we  measure 
the  angle  at  which  the  reflection  of  a  given  set  of  rays 
occurs  most  readily,  and  if  we  know  the  structure  of  the 
crystal,  we  can  calculate  the  wave-length.  Conversely, 

[191] 


X  RAYS 


[Sec.  55 


using  rays  of  known  wave-length,  we  can  deduce  the 
structure  of  a  crystal  with  the  constitution  of  which  we 
are  not  familiar.  (See  Figure  27.) 


Q  ------- 


Fig.  27 
STRUCTURAL  PLAN  OF  A  SIMPLE  CRYSTAL 

This  drawing  represents  the  structure  of  a  crystal  of  potassium  chloride, 
a  substance  similar  to  ordinary  salt,  as  deduced  from  its  action  upon  X  rays. 
The  dark  spheres  represent  chlorine  atoms,  the  light  ones  atoms  of  po- 
tassium. It  will  be  seen  that  the  unit  of  structure  of  the  crystal  is  the 
individual  atom,  since  all  of  the  atoms  are  equidistant  from  their  imme- 
diate neighbors.  For  the  sake  of  clearness,  the  spaces  between  the  atoms 
have  been  exaggerated,  as  compared  with  their  diameters. 

There  are  always  a  number  of  different  ways  in  which 
geometrical  plane  surfaces  can  be  drawn  through  the 
atoms  in  a  crystal,  and  each  of  these  theoretical  surfaces 
is  capable  of  reflecting  or  diffracting  X  rays.  The  result 
is  that  if  a  beam  of  rays  is  sent  into  a  crystal,  it  is  partly 

[  192  ] 


Sec.  66]  LIFE  AND   CATALYSIS 

split  up  into  secondary  beams  which  take  different  direc- 
tions, characteristic  of  the  inherent  planes  of  the  crystal 
atoms.  When  these  latter  beams  are  caught  upon  a 
photographic  plate  a  pattern  is  produced  from  which  the 
constitution  of  the  crystal  can  be  inferred. 

REFERENCES 

Concerning  the  X  rays  see:  C.  W.  C.  Kaye's  "X  Rays"  (1914); 
and  W.  H.  and  W.  L.  Bragg's  "X  Rays  and  Crystal  Structure," 
(1915).  Also:  S.  P.  Thompson's  "Radiation"  (1898),  Chapter  III. 
Bragg's  arguments  with  reference  to  the  corpuscular  properties  of 
the  rays  are  given  in  Norman  Campbell's  "Modern  Electrical 
Theory,"  second  edition  (1913),  pp.  292-304. 


Section  56 
LIFE  AND   CATALYSIS 

Living  bodies  are  complex  mixtures  of  active  chemical 
substances.  These  substances  are  constantly  reacting 
with  each  other  and  as  a  consequence  the  bodies  in 
question  would  soon  be  destroyed  if  it  were  not  for  the 
fact  that  the  chemical  changes  are  so  organized  and  con- 
trolled as  to  ultimately  bring  compensation  for  the  de- 
struction which  they  cause. 

This  control  and  regulation  which  is  so  characteristic 
of  the  activities  of  living  beings  probably  depends  in  the 
last  analysis  upon  a  purely  chemical  principle  called 
catalysis.  This  principle  implies  that  the  mere  presence, 
in  chemical  mixtures,  of  very  minute  quantities  of  cer- 
tain substances,  can  determine  the  nature  of  the  changes 
which  take  place  in  these  mixtures.  Controlling  sub- 
stances of  this  sort  are  called  catalyzers.  The  effects 
which  they  produce  can  be  explained  in  terms  of  atoms, 
molecules  and  electrons. 

[193] 


LIFE   AND   CATALYSIS  [Sec.  66 

The  catalyzers  which  control  the  life  processes  in  dif- 
ferent organisms  are  characteristic  of  these  organisms, 
and  are  undoubtedly  transmitted  to  them  from  their 
progenitors  through  the  germ-cells  from  which  they 
originally  developed.  The  reason  why  certain  catalyzers 
and  not  others  have  been  constantly  transmitted  from 
parent  to  offspring  through  many  generations  is  to  be  found 
in  their  special  power  to  regulate  the  chemical  changes  in 
organisms  so  as  to  permit  the  survival  of  the  species.  In 
other  words,  the  present  physical  and  chemical  structure 
of  organism  must  be  explained  not  only  in  terms  of  atoms 
and  molecules  but  also  in  terms  of  the  history  of  living 
matter  upon  the  earth. 

The  most  important  elements  hi  the  constitution  of 
living  organisms  are  carbon,  hydrogen,  oxygen,  and 
nitrogen,  although  many  others  are  essential.  These 
elements  are  combined,  usually,  to  form  "colloidal" 
systems  of  particles  (see  Section  3),  and  many  of  the 
fundamental  peculiarities  of  living  things  depend  upon 
those  of  colloids. 

Organic  catalyzers  are  called  enzymes,  and  on  the 
above  theory,  enzyme  action  explains  the  mystery  of 
heredity. 

REFERENCES 

See  L.  T.  Troland's  "The  Chemical  Origin  and  Regulation  of 
Life,"  in  the  Monist  for  January,  1914. 

END  OF  PART  TWO 


[194] 


INDEX 


NOTE:    Page  numbers  in  italics  refer  to  the  more  extensive 
discussions  of  a  given  subject. 


Absolute  zero,  20,  107 

,  no  chemical  action  at,  29 

,  state  of  atoms  at,  55 

Absorption  of  light,  30 
jEther,  present  status  of,  146 
Alcohol,  formula  of,  80 
Allotropism,  84,  88 
Alpha  rays,  34,  35,  168 

,  counting  particles  in,  56 

,  origin  of,  45 

,  proved  by  Rutherford  to  be 

helium  atoms,  169-170 

,  scattering  of,  67,  176 

Argon,  138 

Aston,  F.  W.,  on  meta-neon,  75 

Atom,  energy  of,  772 

— ,  internal  forces  of,  10 

— ,  nucleus,  size  of,  176-177 

— ,  nucleus  theory  of,  44-45,  71,  74, 

776-778 

— ,  openwork  structure  of,  42 
— ,  permanence  of,  10 
— ,  Saturnian,  61,  182 

,  radiation  from,  181-182 

— ,  single,  effect  due  to,  35 

— ,  solar  system  theory  of,  42 

— ,  structure  of,  41-43,  43-46,  174- 

782.    (See  Atomic  structure) 
— ,  Thomson's  theory  of  structure, 

174 

— ,  unit  of  crystal  structure,  113 
Atomic  heats,  118 

—  magnitudes,  58 

-  numbers,  46,  179-180 

—  structure,  recent  discoveries  con- 

cerning, 43-46 
—  and  spectra,  155 


Atomic  volumes,  3-4 

and  atomic  weights,  67 

—  weights  and  atomic  numbers,  46 
,  irregularity  of,  75 

,  methods  of  determining,  65 

,  table  of,  62-€3 

Atoms,  2 

—  and  electrons,  relations  between, 

126-128 

—  and  life,  50-51 

—  and  molecules,  relative  sizes  of, 

(Fig.  1),  3 

—  and  positive  electricity,  23 

— ,  arrangement  in  molecule,  76-86 
— ,  attraction  of  identical,  138 

-  neutral,  (Fig.  22),  138 
— ,  charges  carried  by,  24 
— ,  density  of,  4 
— ,  individuality  of,  5 
— ,  kinds  of,  2-3 
— ,  number  in  unit  volume,  54,  56 

—  of  a  liquid,  (Fig.  5),  opposite  page 

6 

—  of  a  solid,  (Fig.  7),  opposite  page 

10 

—  of  light,  183 

— ,  shape  of,  2,  60-61 
— ,  sizes  of,  2 

,  methods  of  finding,  53-58 

— ,  species  of,  62 

— ,  spontaneous  disruption  of,  10 

— ,  structures  of,  and  periodic  table, 

70 

— ,  tendency  to  form  groups,  5 
— ,  visibility  of,  58-59 
Attraction,  forces  of,  within  bodies, 

97 
Avogadro,  principle  of,  66, 107 


[196] 


INDEX 


B 

Balmer's  formula,  181 
Battery,  electric,  action  of,  25 
Becquerel,  Henri,  52,  165 
Benzene,  derivatives  of,  (Fig.  13), 

81-82 

— ,  ring  formula  of,  80-83 
Beta  rays,  34,  168 
-  and  X  rays,  189 
— ,  from  potassium,  173 

,  origin  of,  45,  177 

,  penetrating  power  of,  35,  42, 

169 
,  relation  to  gamma  rays,  38, 

171 

— ,  scattering  of,  67 
Black  body,  radiation  from,  152 
Bohr's  theory  of  atomic  structure, 

181 
Boiling  points,  105 

— ,  effect  of  ionization  on,  140- 

141 

Boyle,  law  of,  106 
Bragg,  W.  H.,  on  corpuscular  prop- 
erties of  X  rays,  190-191 
Brownian   movement,  16-17,  110- 

111 
,  path  of  particle  in,  (Fig.  16), 

100 


Campbell,  Norman,  on  radio-activ- 
ity of  potassium,  173 

Carbon,  allotropic  forms  of,  85 

— ,  compounds  of,  77 

Catalysis,  145 

-  and  life,  793-794 

Cathode  rays,  120 

,  action  of,  (Fig.  18),  121 

— ,  action  of  magnet  on,   (Fig. 
19),  122,  163 
—  and  X  rays,  189 

Cell,  electric,  145 

Cells,  149 

—  and  atoms,  50-51 

Chances,  molecular,  and  averages, 

93 

Charles,  law  of,  16,  106 
Chemical  affinity,  91,  736-739 

—  action,  7 

and  electrons,  27-28 


Chemical   affinity   and    ionization, 

126 
,  molecular  basis  of,  142-144 

—  change,  effects  and  conditions  of, 

144-146 
,  mystery  of,  88 

—  energy,  146 

,  in  relation  to  radio-activity, 

172 

—  formulae,  77 

—  properties  of   atom,  determina- 

tion of,  45,  89 

—  reactions,  reversibility  of,  143 
,  velocity  of,  143 

—  valency,  141-142 
Chlorine,  atomic  weight  of,  65 
Coagulation,  59 

Cohesion,  forces  of,  11,  92,  172 

Cold,  nature  of,  20 

Colloids,  59,  112 

-  and  life,  194 

Color,  due  to  reflection,  158 

—  mixture,  and  white  light,  158 
— ,  physical  basis  of,  87,  757-759 
— ,  sensations  of,  158 
Compounds,  6 

— ,  properties  of,  86 
Conduction,  electrical.   (See  Elec- 
trical conduction) 

—  of  heat.      (See   Heat    conduc- 

tion) 
Conductivity,    electrical,   basis   of, 

130 

— ,  of  gases,  98 
Cooling,  due  to  evaporation,  19 
Copper,  color  of  compounds  of,  88 
Critical  points  of  liquids,  105 
Crystal,  as  a  unit  of  structure,  112 
— ,  fixity  of  molecules  in,  103 
— ,  planes  of,  192-193 
— ,  structural  plan  of  simple,  (Fig. 

27),  192 
—  structure   and   molecular  form, 

85 
and  X  rays,  49,  53,  773,  114, 

797-793 

Crystalline  state,  112 
Crystals,  liquid,  19,  112,  114 
Curie,  M.  and  Mme.,  165 
Current,  electrical.   (See  Electrical 

current) 


[196] 


INDEX 


Dalton,  John,  52 

Diamond,  88 

Dielectric  capacity  and  electrolytic 

dissociation,  139-140 
and  index  of  refraction,  160- 

161 
Diffraction,  191-192 

—  of  X  rays,  113,  191 
Diffusion,  98,  99-101 

—  and  atomic  size,  57 

—  paths,  (Fig.  16),  100 
Dispersion,  of  colloids,  59 

—  of  light,  laws  governing,  160 
Distribution  curve  of  temperature 

radiation,  (Fig.  25),  153,  757- 

752 
Du  Long  and  Petit,  on  atomic  heats, 

118 
Dynamo,  action  of,  25,  33 


£ 

Elasticity,  87 

Electrical  conduction,  in  gases,  737- 

732 
— ,  in  liquids,  131-132 

—  current,  direction  of,  131 
,  effects  connected  with,  729- 

737 

,  nature  of,  24 

— ,  produced  by  chemical 
change,  145 

—  force  lines,  kinks  in,  147 

— ,  nature  of,  146-147 

—  forces,  importance  in  nature,  724 

—  lamp,  principle  of,  25 

—  motor,  principle  of,  32 

—  power  transmission,  26,  733 

—  resistance,  nature  of,  129,  130 

—  waves,  absorption  of,  30 
,  generation  of,  30 

— ,  reflection  of,  31 
Electricity,  conduction  in  gases,  93 
— ,  conduction  through  liquids,  140 
— ,  in  all  bodies,  23 
— ,  laws  of  attraction  and  repulsion, 

22 

— ,  positive  and  negative,  22 
— ,  positive,   form  in   Thomsonian 

atom,  175 


Electricity,  relation  to  magnetism, 

32 

— ,  "  speed  "  of,  24,  25 
Electrolysis,  737-732,  139-141 
— ,  deposition  by,  55 
Electrolytic    dissociation,    739-747 
Electron,  and  its  behavior,  27-27 
— ,  charge  of,  23,  120,  122,  123 
— ,  contains    only    negative    elec- 
tricity, 124 
— ,  density  of,  22 
— ,  discovery  of,  52,  720 
— ,  flattening  at  high  speed,  124 
— ,  mass  of,  120,  123 
— ,  measurement  of,  120-124 
— ,  natural  unit  of  electricity,  55 
— ,  radiation  from,  46 
— ,  shape  of,  22,  124 
— ,  size  of,  21,  123 
— ,  structure  of,  22,  724 
— ,  weight  of,  22 
— ,  Zeeman  effect,  29,  149 
Electrons,  2 

— ,  action  of  magnetic  field  on,  129 
— ,  affinity  of  atoms  for,  736-739 
— ,  affinity  of  elements  for,  26 
—  and  chemical  action,  27,  28,  29 

—  and    ions,    reactions    between, 

125-128 

—  and  light  waves,  29,  147 

—  and  line  spectra,  155 

—  and  magnetism,  32-34,  164 

—  and  selective  absorption  of  light, 

158 

—  and  temperature  radiation,  150- 

151 

—  and  valency,  142 

— ,  arrangement  in  atom,  175 

— ,  as  beta  rays,  34-35 

— ,  emitted  by  metals  under  action 

of  light,  184 

— ,  evaporation  of,  27,  134 
— ,  "  free,"  26,  130 

— ,  and  heat  conduction,  110 
— ,  in  atom,  42-46,  175,  178 

,  two  groups  of,  177 

— ,  in  the  electric  current,  24 
— ,  moving,  deflection  by  magnet- 
ism, 32, 762-763 
— ,  number  in  atom,  178 
— ,  position  in  atom,  44,  45. 


[197] 


INDEX 


Electrons,  rings  of,  in  atom,  175 

— ,  valency,  89,  177 

Elements,  5,  6 

— ,  affinity  for  electrons,  26 

— ,  chemical,  as  atomic  mixtures,  74 

,  number  possible,  180 

— ,  electro-positive    and    negative, 

26,  136-138 
— ,  evolution  of,  41 
— ,  inert,  explanation  of,  138-139 
— ,  life  of,  41 
— ,  molecules  of,  83 
— ,  periodic  table  of,  68-76 
— ,  primitive,  in  hottest  stars,  174 
— ,  properties  of,  and  periodic  table, 

69 
— ,  radio-active  series  of,  (Fig.  11), 

36-37 

— ,  radio-activity,  alleged  of  all,  40 
— ,  specific  nature  of,  62-63 
— ,  systematic  relations  of,  68 
— ,  table  of,  62-63 
— ,  undiscovered,  70 
Emulsions,  59 
Energy,  2 

— ,  atomic  nature  of,  48 
— ,  chemical,  146 

— ,  equipartition  of,  95-96,  111,  118 
— ,  intra-atomic,  39,  172 
— ,  kinetic,  95 
— ,  given  off  by  radium,  40 
Entropy,  188 
Enzymes,  194 
Equations,  chemical,  90 
Equilibrium,  chemical,  143-144 
Equipartition  of  energy,  95-96,  111, 

118 

Ether,  29 
— ,  methyl,  78 
Evaporation,  13 
— ,  cooling  effect  of,  19 
Evolution,      inorganic,      Lockyer's 

theories,  173-174 
— ,  organic,  194 
Expansion,  due  to  heat,  18 


Fluids,  motion  of  particles  through, 

56 

Fluorescence,  185 
Fog,  condensation  around  ions,  122, 

123,  126 

Forces,  inter-atomic,  91 
— ,  intra-atomic,  91 
— ,  magnetic,  direction  of,  around 

moving   electric   charge,    (Fig. 

26),  162 

Formulae,  chemical,  77-75 
— ,  graphical,  meaning  of,  78 
— ,  structural,  77 
Free  electrons,  26 

—  and    electrical    conductivity, 

130 

and  heat  conduction,  110 

Frequency,    and  energy     of    light 

quantum,  185 
Freezing  points,  87 

— ,  effect  of  ionization  on,  140- 

141 

Friction,  causes  heat,  19 
— ,  molecular  explanation  of,  12 
Fusion,  latent  heat  of,  103 


Gamma  rays,  34,  35,  36,  168 

,  nature  of,  /  70-1 71 

— ,  origin  of,  45 
— ,  relation  to  beta  rays,  38 
— ,  wave-length  of,  156 
Gas  law,  108 
— ,  model  of  a,  14-16 
—  pressure,  cause  of,  15,  16 
Gases,  atomic  heats  of,  119 
— ,  conduction  of  electricity  through, 

131-132 

— ,  emission  of  light  by,  152,  153 
— ,  simple  laws  of,  106-108 
Gay-Lussac,  principle  of,  107 
Geiger  and  Nuttall,  on  life  of  radio- 
elements,  167 
Gold-leaf,  molecular  thickness  of, 

54 
Gravitation,  91 


F 

Families,  in  periodic  table,  69 
Faraday,  Michael,  52 
Films,  thin,  54 


Hall  effect,  129 
Hardness,  87 
[198] 


INDEX 


Heat  and  allied  phenomena,  II- 
21 

—  and  chemical  action,  28,  145 

—  conduction,  16,  57,  109-110 

and  electrical  conduction,  130 

— ,  due  to  electric  current,  25 

— ,  due  to  friction,  19 

—  energy,  118 

in  bodies,  amount  of,  20-21 

—  motion,  visibility  of,  16 
— ,  radiant,  13,  29-30,  102 

— ,  produced  by  chemical  change, 
144 

—  wave,  156 
Heats,  latent,  102-105 
Helium,  138 

— ,  in  alpha  rays,  35 

,  Rutherford's  experi- 
ment, 170 

— ,  in  atom  structure,  71 

— ,  in  stars,  174 

Heredity,  194 

Hertz,  H.  R.,  52 

Hertzian  waves,  29,  48,  156 

Hydrocarbons,  five  isomeric,  (Fig. 
12),  79 

Hydrogen,  96 

— f  atom,  structure  of,  71,  74,  180- 
181 

—  ion,  nature  of,  180 
Hysteresis,  165 


Index  of  refraction,  160 

Interference,  of  X  rays,  113,  191 

Intra-atomic  and  inter-atomic 
forces,  91 

Inverse  squares,  law  of,  for  radia- 
tion, 183 

lonization,  126 

— ,  energy  of,  126 

—  of  substances  dissolved  in  water, 
139 

Ions,  atoms  and  electrons,  forces 
between,  (Fig.  20),  127 

— ,  conduction  of  electricity  by, 
131-132 

— ,  how  produced,  125-126 

— ,  motion  through  solutions,  140 

Isomerism,  stereo-,  84-86 


Isomers,  and   structural    formulae, 

77-80 

Isotopes,  45,  53,  71-75,  89 
— ,  table  of,  64 
Isotopism,  178-179 


Jeans,  J.  H.,  on  quantum  theory, 
187 


Kinetic  energy,  95 

Kinetic    molecular    theory,   92-9^, 

115 
Kleeman,  R.  D.,  on  atomic  shapes, 

61 


Latent  heats,  102-105 

Laue,  M.,  experiments  on  X  rays, 

49 

Lenard,  P.  A.,  52 
Lenses  and  prisms,  action  of,  160 
Life  and  atoms,  50 

—  and  catalysis,  59 

—  and  colloids,  59 

Light,  absorption  of,  30,  157-158 

—  and  chemical  action,  28 

—  and  electronic  vibration,  29 

—  and  magnetic  field,  34, 149 

— ,  conditions  of  production,   150- 
155 

emission  by  gases,  152,  1 54 

frequency  of,  157 

ionization  due  to,  126 

polarized,  86 

produced  by  chemical  change, 
145 

reflection  of,  31,  157 

refraction  of,  159-161 

selective  absorption  of,  157 

ultra-violet,  156 

velocity  of,  156 

,  in  different  media,  31 

— ,  wave-length  of,  156 

—  waves  and  electrical  force-lines, 

146-148 

Lines  of  force,  electrical,  146-147 
Line  spectra,  154 

—  and  structure  of  the  atom, 
42,43 


[199] 


INDEX 


Liquid,  model  of  a,  17-18 

—  state,  13 

Liquids,    conduction   of   electricity 

in,  131-132 
Lockyer,  Sir  Norman,  on  inorganic 

evolution,  173-174 
Loreutz,  H.  A.,  52 


M 
Magnetic  field,  due  to  motion  of 

electricity,  161-162 
Magnetism,  163-165 
— ,  dia-  and  para-,  163 
— ,  permanent,  33,  164 
— ,  relation  to  electricity,  32,  767- 

762 

Magnetization,  mechanism  of,  164 
Malleability,  and  temperature,  103 
Mass  action,  law  of,  in  chemistry, 

142-143 
Matter,  history  of  modern  theory 

of,  52-53 

Maxwell,  J.  C.,  52 
Mean    free    path    (of    molecules), 

97-99 

—  and     electrical     conduc- 
tivity, 130-131 
— ,  properties  dependent  on, 

98 
Mechanics,   fundamental  laws   of, 

92,  93 

— ,  laws  of,  and  atomic  events,  787- 

758 

— ,  statistical,  93,  94,  97 
Melting,  13,  14 

—  points,    87 

Membrane,  semi-permeable,  109 

Mendelejeff,  and  periodic  table,  70 

Mercury  vapor,  83-84 

Metals,  heat  conduction  in,  110 

— ,  thermo-electric  series  of,  135 

Meta-neon,  75 

Mica,  112-113 

Millikan,    R.    A.,    experiments    of 

quantum  theory,  48 
Molecular  action,  and  probability, 

93,  94 

—  activities,  individuality  of,  94 

—  chaos,  142-143 

—  forces,  internal,  86-87 


Molecular  motion,  speeds  of,  94-97 

—  speeds,  distribution    curve    of, 

(Fig.  17),  116 

,  distribution  law  of,  775-777, 

151 

—  theory,    kinetic.     (See    Kinetic 

molecular  theory) 

—  volumes,  and  gas  law,  108 

—  weights,  66 

Molecule,    arrangement   of   atoms 

in,  76-86 

— ,  motion  between  impacts,  97-99 
Molecules,  5,  6 

—  and    visible    particles,    relative 

sizes  of,  (Fig.  2),  4 
— ,  electrical  constitution  of,  (Fig. 

10),  28 

— ,  frictionless,  12,  13 
— ,  gas,  motion  of,  14 
— ,  mean  free  path  of,  97-99 
— ,  motion  of,  77-27 

—  of  a  gas,  (Fig.  8),  opposite  page 

12 

—  of  steam,  (Fig.  4),  opposite  page 

6 

—  of  water,  (Fig.  3),  opposite  page  4 
— ,  structure  of,  and  crystals,  85 
Moseley,  H.  G.  J.,  on  atomic  num- 
bers, 180 

Motion,  laws  of,  92,  93 
Motor,  electric,  principle  of,  32 


Neon,  and  meta-neon  (Isotopes),  75 

Nernst,  W.,  on  specific  heats,  186 

Newton,  laws  of,  93 

Niton,  166 

Non-conductors,  electrical,  130 


Oil  films,  54 

Organic  compounds,  76 

— ,  formulae,  (Fig.  6),  8-9 
Osmotic  pressure,  108-109,  144 
Oxygen,  atomic  weight  of,  66 


Periodic  table  of  the  elements,  68- 
76 


[200] 


INDEX 


Periodic  table  and  atomic  numbers, 
46 

,  blanks  in,  70 

,  defects  in,  70 

,  significance  of,  70 

,  Thomson's  explanation  of, 

175 

Periods,  in  periodic  table,  69 

,  and  electron  rings, 

175 

Perrin,  J.,  on  the  Brownian  move- 
ment, 110 

Phosphorescence,  185 

Photo-electric  effect,  184 

Physical  properties,  basis  of,  87 

Planck,  Max,  on  quantum  theory, 
46,  187 

Positive  electricity,  form  in  Thom- 
sonian  atom,  175 

Potassium  chloride,  crystal  struc- 
ture of,  113 

— ,  radio-activity  of,  40,  41,  773 

Power,  electrical  transmission  of, 
26,  133 

Pressure,  of  a  gas,  16,  106-107 

— ,  osmotic,  108-109 

Prism,  action  of,  on  light,  155 

Probability  and  entropy,  188 

—  and  molecular  action,  93,  94 

Protyle,  71 

Prout's  hypothesis,  71 

Pyrometer,  optical,  151 


Quantum  theory  of  radiant  energy, 
46-49,  53, 147-148,  182-189 
—  and  specific  heats,  119-120, 
186 

and  hydrogen  atom,  181 

and    temperature    radiation, 

152,  187 

— ,  difficulties  with,  48-49 
— ,  significance  of,  187 


Radiant   energy,   similarity   of   all 

forms,  49 
Radiation,  due  to  acceleration  of  an 

electron,  (Fig.  23),  148 
— ,  due  to  temperature,  150-152 


Radiation,    quality    dependent   on 

temperature,  47 
— ,  recent   discoveries   concerning, 

46-50 

Radio-active  substances,  165-168 
Radio-activity,  34-41 

—  and  atom  structure,  44 
— ,  cause  of,  38 

— ,  energy  of,  39-40 

— ,  general  property  of  matter,  40, 

166 

— ,  laws  of,  166,  167 
— ,  not  chemical,  39 
— ,  penetrating  power  of  rays,  124 
Radio-elements,  40 
— ,  chemical  diversity  of,  179 
— ,  discovery  of,  165 
— ,  isotopes  among,  74 
— ,  life  of,  166 
— ,   position  of,  in  periodic  table, 

167 

— ,  successive  disruptions  of,  38 
— ,  table  of,  64 
Radium,  34 
— ,  atomic  weight  of,  40 

—  atom,  stability  of,  172 
— ,  discovery  of,  165 

— ,  rays  from,  study  of,  168-169 
Rayleigh,  Lord,  radiation  law,  47 
Rays,  alpha.     (See  Alpha  rays) 
— ,  beta.    (See  Beta  rays) 
— ,  cathode.    (See  Cathode  rays) 
— ,  gamma.   (See  Gamma  rays) 
— ,  X.    (See  X  rays) 
Reactions,  chemical,  kinds  of,  90 
Reflecting  power,  basis  of,  158 
Reflection,  diffuse,  31 

—  of  light,  31 

—  of  X  rays,  49 
Refraction,  index  of,  160 

—  of  light,  31 

— ,  relation  to  absorption  of  light, 

161 
Resistance,  electrical,  98 

— ,  nature  of,  129-130 
Retina,  the,  158 
Roentgen,  W.  C.,  53 
Rowland's  experiment,  161 
Rutherford,  Ernest,  53 

— ,  on  helium  in  alpha  rays,  769- 
770 


[201] 


INDEX 


Rutherford,   Ernest,   on   scattering 
of  alpha  rays,  176 


Schmidt,  G.  K.,  165 

Secondary  rays,  171 

Series,  disintegration,  of  elements, 
165 

— ,  thermo-electric,  135 

Soap-bubble  films,  54 

Solid  and  crystalline  states,  112- 
115 

—  and  liquid,  stages  between,  114 

— ,  liquid,  and  gaseous  states,  13 

— ,  model  of  a,  18 

Solution,  and  electrical  decomposi- 
tion, 139-141 

— ,  simple  laws  of,  106-108 

Sound,  13,  101-102 

Specific  heats,  118-120 

— ,   at  low   temperatures,    119, 
186 

Spectra,  line,  755 

— ,  varying  complexity  of,  173 

Spectral  lines,  42,  43 

Spectrum,  normal,  156 

Stark  effect,  150 

Stark,  J.,  150 

Statistical  mechanics,  93,  94 

Stokes'  law,  185 

Stoney,  G.  J.,  52 

Sugar,   constitution    of    molecule, 
6,7 

Sugars,  formulae  and  crystals  of,  85, 
86 

Sulphur,  allotropic  forms  of,  84 

Surface  tension,  104,  117 


Tartaric  acid,  crystals  of,  (Fig.  15), 

85 
— ,  molecular  structure  of,  (Fig. 

14),  84 
Temperature,  and  molecular  energy, 

94-95 

—  and  radiation,  47 

—  radiation,  150-152 

,  and  quantum  theory,  187 

Temperatures,    low,    and    specific 
heats,  119,  120 


Thomson,  Sir  J.  J.,  52 

— ,  and  meta-neon,  75 
,  discovery    of    the    electron, 

120-124 
,  theory    of    atom    structure, 

774-775 
Thermodynamics,   second   law   of, 

188 
Thermo-electric  circuit,   (Fig.  21), 

134 

Thermo-electricity,  133-136 
Thermo-electric   series   of  metals, 

135 

Thermopile,  principle  of,  133-134 
Transparency,  theory  of,  30 
Traube,  J.,  on  atomic  volumes,  67 
Triads,  of  elements,  68 


Ultra-microscope,  58 
Ultra-violet  light,  156 
Uranium,  life  of,  167 


Vacuum  tubes,  98 
Valency,  chemical,  141-142 
— ,  in  periodic  table,  69 

—  electrons,  89,  177 

Van  der  Waals,  gas  formula  of,  57, 

108 
Vapor,  above  liquid,  temperature  of, 

117 
— ,  molecules  of,  at  liquid  surface, 

(Fig.  9),  15 

—  pressure,  775-777 

,  laws  of,  116 

Vaporization,  latent  heat  of,  104 
Viscosity,  of  gases,  57 
Voltage,  26,  30 

W 

Water,  decomposition  of,  7,  66,  90 
Waves,  electro-magnetic,  gamut  of, 

50,  755-757 

— ,  Hertzian,  29,  48,  156 
Weights,     atomic.      (See     Atomic 

weights) 
Wien's  law,  47 
Wilson,  C.   T.   R.,   photography  of 

ionization  paths,  126,  169 


[202] 


INDEX 


X  rays,  184,  185,  186 

—  and  atomic  numbers,  180 

—  and  crystal  structure,  113 

,  characteristic,  189-190 

,  corpuscular     properties     of, 

190-191 

— ,  diffraction  of,  191-193 

,  discovery  of,  53 

,  measurement  of,  189-193 

— ,  nature  of  49,  189-190 

,  penetrating  power  of,  190 

,  reflection  of,  49,  19 1-193 


X  rays,  similarity  of  gamma  rays, 
171 

,  spectra,  source  of,  in  atom. 

178 

—  spectrometer,  190 
— ,  wave-length  of,  156 


Zeeman  effect,  29,  34,  149-150, 155, 

(Fig.  24),  150 
— ,  Paul,  149 
Zero,  absolute,  20,  107 


[203] 


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